diff --git a/implementations/Crafter.Review-Platform.cpp b/implementations/Crafter.Review-Platform.cpp
new file mode 100644
index 0000000..4ed6cdc
--- /dev/null
+++ b/implementations/Crafter.Review-Platform.cpp
@@ -0,0 +1,415 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+module;
+#include <fcntl.h>
+#include <pwd.h>
+#include <sys/file.h>
+#include <sys/stat.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <cerrno>
+#include <cstdlib>
+#include <ctime>
+
+module Crafter.Review:Platform_impl;
+import :Platform;
+import :Json;
+import :Text;
+import :Crypto;
+import std;
+
+namespace fs = std::filesystem;
+
+namespace Crafter::Review::Platform {
+    namespace {
+        Mode CurrentModeValue = Mode::Native;
+
+        SystemPaths& PathsValue() {
+            static SystemPaths paths = [] {
+                SystemPaths defaults;
+                if (const char* runtime = std::getenv("XDG_RUNTIME_DIR"); runtime && *runtime) {
+                    defaults.runtimeDir = runtime;
+                } else {
+                    defaults.runtimeDir = std::format("/run/user/{}", getuid());
+                }
+                return defaults;
+            }();
+            return paths;
+        }
+
+        // [section] key=value, as GLib's key files are written.
+        std::optional<std::string> KeyFileValue(std::string_view text, std::string_view section, std::string_view key) {
+            bool inSection = false;
+            for (const std::string& line : Dyn::SplitLines(text)) {
+                const std::string_view view(line);
+                if (view.starts_with('[')) {
+                    inSection = view == std::format("[{}]", section);
+                    continue;
+                }
+                if (!inSection) continue;
+                const std::size_t equals = view.find('=');
+                if (equals == std::string_view::npos) continue;
+                if (Dyn::Strip(view.substr(0, equals)) == key) return Dyn::Strip(view.substr(equals + 1));
+            }
+            return std::nullopt;
+        }
+
+        std::optional<std::int64_t> ToInt(std::string_view text) {
+            std::int64_t value = 0;
+            const auto [end, error] = std::from_chars(text.data(), text.data() + text.size(), value);
+            if (error != std::errc() || end != text.data() + text.size()) return std::nullopt;
+            return value;
+        }
+    }
+
+    void Init() {
+        CurrentModeValue = fs::exists(PathsValue().flatpakInfo) ? Mode::Sandbox : Mode::Native;
+        if (CurrentModeValue == Mode::Sandbox) RestoreHostXdg();
+    }
+
+    Mode CurrentMode() {
+        return CurrentModeValue;
+    }
+
+    bool InSandbox() {
+        return CurrentModeValue == Mode::Sandbox;
+    }
+
+    void SetMode(Mode mode) {
+        CurrentModeValue = mode;
+    }
+
+    const SystemPaths& Paths() {
+        return PathsValue();
+    }
+
+    void SetPaths(SystemPaths paths) {
+        PathsValue() = std::move(paths);
+    }
+
+    std::optional<std::string> GetEnv(std::string_view name) {
+        const std::string key(name);
+        const char* value = std::getenv(key.c_str());
+        if (value == nullptr) return std::nullopt;
+        return std::string(value);
+    }
+
+    std::map<std::string, std::string> Environment() {
+        std::map<std::string, std::string> out;
+        for (char** entry = environ; entry != nullptr && *entry != nullptr; ++entry) {
+            const std::string_view text(*entry);
+            const std::size_t equals = text.find('=');
+            if (equals == std::string_view::npos) continue;
+            out.emplace(std::string(text.substr(0, equals)), std::string(text.substr(equals + 1)));
+        }
+        return out;
+    }
+
+    std::string Home() {
+        if (const std::optional<std::string> home = GetEnv("HOME"); home && !home->empty()) return *home;
+        if (const passwd* entry = getpwuid(getuid()); entry != nullptr && entry->pw_dir != nullptr) return entry->pw_dir;
+        return "/";
+    }
+
+    void RestoreHostXdg() {
+        const std::string home = Home();
+        const std::array<std::pair<std::string_view, std::string_view>, 4> defaults = { {
+            { "CONFIG", ".config" },
+            { "CACHE", ".cache" },
+            { "DATA", ".local/share" },
+            { "STATE", ".local/state" },
+        } };
+        for (const auto& [kind, relative] : defaults) {
+            const std::string name = std::format("XDG_{}_HOME", kind);
+            const std::optional<std::string> host = GetEnv(std::format("HOST_XDG_{}_HOME", kind));
+            const std::string value = host && !host->empty() ? *host : Dyn::JoinPath(home, relative);
+            setenv(name.c_str(), value.c_str(), 1);
+        }
+    }
+
+    fs::path SelfExe() {
+        std::error_code error;
+        const fs::path exe = fs::read_symlink("/proc/self/exe", error);
+        return error ? fs::path() : exe;
+    }
+
+    double Now() {
+        const auto since = std::chrono::system_clock::now().time_since_epoch();
+        return std::chrono::duration<double>(since).count();
+    }
+
+    std::string LocalTime(std::string_view format, double epochSeconds) {
+        const std::time_t seconds = static_cast<std::time_t>(std::floor(epochSeconds));
+        std::tm local {};
+        localtime_r(&seconds, &local);
+        std::array<char, 128> buffer {};
+        const std::string pattern(format);
+        const std::size_t length = std::strftime(buffer.data(), buffer.size(), pattern.c_str(), &local);
+        return std::string(buffer.data(), length);
+    }
+
+    std::string LocalTime(std::string_view format) {
+        return LocalTime(format, Now());
+    }
+
+    std::string TodayIso() {
+        return LocalTime("%Y-%m-%d");
+    }
+
+    std::optional<std::string> ReadFile(const fs::path& path) {
+        std::ifstream in(path, std::ios::binary);
+        if (!in) return std::nullopt;
+        std::string bytes((std::istreambuf_iterator<char>(in)), std::istreambuf_iterator<char>());
+        if (in.bad()) return std::nullopt;
+        return bytes;
+    }
+
+    void WriteAtomic(const fs::path& path, std::string_view content, std::uint32_t mode) {
+        if (path.has_parent_path()) fs::create_directories(path.parent_path());
+        const fs::path temp = std::format("{}.{}.tmp", path.string(), PublishedPid());
+        const std::int32_t fd = open(temp.c_str(), O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC, static_cast<mode_t>(mode));
+        if (fd < 0) throw std::runtime_error(std::format("{}: {}", temp.string(), Dyn::ErrnoText(errno)));
+        std::size_t written = 0;
+        while (written < content.size()) {
+            const ssize_t step = write(fd, content.data() + written, content.size() - written);
+            if (step < 0) {
+                if (errno == EINTR) continue;
+                const std::int32_t failure = errno;
+                close(fd);
+                unlink(temp.c_str());
+                throw std::runtime_error(std::format("{}: {}", temp.string(), Dyn::ErrnoText(failure)));
+            }
+            written += static_cast<std::size_t>(step);
+        }
+        close(fd);
+        if (rename(temp.c_str(), path.c_str()) != 0) {
+            const std::int32_t failure = errno;
+            unlink(temp.c_str());
+            throw std::runtime_error(std::format("{}: {}", path.string(), Dyn::ErrnoText(failure)));
+        }
+    }
+
+    FileLock::FileLock(const fs::path& lockPath) {
+        std::error_code error;
+        if (lockPath.has_parent_path()) fs::create_directories(lockPath.parent_path(), error);
+        fd = open(lockPath.c_str(), O_WRONLY | O_CREAT | O_APPEND | O_CLOEXEC, 0644);
+        if (fd < 0) {
+            problem = Dyn::ErrnoText(errno);
+            return;
+        }
+        while (flock(fd, LOCK_EX) != 0) {
+            if (errno == EINTR) continue;
+            problem = Dyn::ErrnoText(errno);
+            close(fd);
+            fd = -1;
+            return;
+        }
+    }
+
+    FileLock::~FileLock() {
+        if (fd >= 0) {
+            flock(fd, LOCK_UN);
+            close(fd);
+        }
+    }
+
+    bool FileLock::Held() const {
+        return fd >= 0;
+    }
+
+    const std::string& FileLock::Problem() const {
+        return problem;
+    }
+
+    Json LockedUpdate(const fs::path& path, const std::function<bool(Json&)>& change, const Json& fallback,
+                      std::uint32_t mode, const std::function<void(const std::string&)>& warn) {
+        const FileLock lock(std::format("{}.lock", path.string()));
+        if (!lock.Held() && warn) {
+            warn(std::format("{}: no lock ({}) - best-effort write", path.filename().string(), lock.Problem()));
+        }
+        Json data;
+        try {
+            if (const std::optional<std::string> text = ReadFile(path)) data = Json::parse(*text);
+        } catch (const std::exception&) {
+            data = Json();
+        }
+        if (!data.is_object()) data = fallback.is_object() ? fallback : Json::object();
+        if (change(data)) {
+            try {
+                WriteAtomic(path, DumpFile(data), mode);
+            } catch (const std::exception& e) {
+                if (warn) warn(std::format("{}: not saved ({})", path.filename().string(), e.what()));
+            }
+        }
+        return data;
+    }
+
+    std::optional<std::int64_t> ProcStart(std::int64_t pid) {
+        const std::optional<std::string> stat = ReadFile(Paths().proc / std::to_string(pid) / "stat");
+        if (!stat) return std::nullopt;
+        const std::size_t close = stat->rfind(')');
+        if (close == std::string::npos) return std::nullopt;
+        const std::vector<std::string> rest = Dyn::Split(std::string_view(*stat).substr(close + 1));
+        if (rest.size() < 20) return std::nullopt;
+        if (rest[0] == "Z" || rest[0] == "X") return std::nullopt;
+        return ToInt(rest[19]);
+    }
+
+    std::string BootId() {
+        const std::optional<std::string> text = ReadFile(Paths().proc / "sys" / "kernel" / "random" / "boot_id");
+        if (!text) return "";
+        return Dyn::Slice(Dyn::Strip(*text), 0, 8);
+    }
+
+    std::map<std::string, std::int64_t> MemInfo(const fs::path& path) {
+        std::map<std::string, std::int64_t> out;
+        const std::optional<std::string> text = ReadFile(path);
+        if (!text) return out;
+        for (const std::string& line : Dyn::SplitLines(*text)) {
+            const std::size_t colon = line.find(':');
+            if (colon == std::string::npos) continue;
+            const std::vector<std::string> parts = Dyn::Split(std::string_view(line).substr(colon + 1));
+            if (parts.empty()) continue;
+            const std::optional<std::int64_t> value = ToInt(parts[0]);
+            if (!value) continue;
+            out[line.substr(0, colon)] = parts.size() > 1 && parts[1] == "kB" ? *value / 1024 : *value;
+        }
+        return out;
+    }
+
+    std::map<std::int64_t, std::vector<std::int64_t>> PpidMap() {
+        std::map<std::int64_t, std::vector<std::int64_t>> out;
+        std::error_code error;
+        for (const fs::directory_entry& entry : fs::directory_iterator("/proc", error)) {
+            const std::string name = entry.path().filename().string();
+            const std::optional<std::int64_t> pid = ToInt(name);
+            if (!pid) continue;
+            const std::optional<std::string> stat = ReadFile(entry.path() / "stat");
+            if (!stat) continue;
+            const std::size_t close = stat->rfind(')');
+            if (close == std::string::npos) continue;
+            const std::vector<std::string> rest = Dyn::Split(std::string_view(*stat).substr(close + 1));
+            if (rest.size() < 2) continue;
+            if (const std::optional<std::int64_t> ppid = ToInt(rest[1])) out[*ppid].push_back(*pid);
+        }
+        return out;
+    }
+
+    std::vector<std::int64_t> Tree(std::int64_t pid, const std::map<std::int64_t, std::vector<std::int64_t>>& kids) {
+        std::vector<std::int64_t> out;
+        std::vector<std::int64_t> todo { pid };
+        std::set<std::int64_t> seen;
+        while (!todo.empty()) {
+            const std::int64_t next = todo.back();
+            todo.pop_back();
+            if (!seen.insert(next).second) continue;
+            out.push_back(next);
+            if (const auto found = kids.find(next); found != kids.end()) todo.insert(todo.end(), found->second.begin(), found->second.end());
+        }
+        return out;
+    }
+
+    double PssMb(std::span<const std::int64_t> pids) {
+        std::int64_t kb = 0;
+        for (const std::int64_t pid : pids) {
+            const std::optional<std::string> text = ReadFile(std::format("/proc/{}/smaps_rollup", pid));
+            if (!text) continue;
+            for (const std::string& line : Dyn::SplitLines(*text)) {
+                if (!line.starts_with("Pss:")) continue;
+                const std::vector<std::string> parts = Dyn::Split(line);
+                if (parts.size() > 1) {
+                    if (const std::optional<std::int64_t> value = ToInt(parts[1])) kb += *value;
+                }
+                break;
+            }
+        }
+        return static_cast<double>(kb) / 1024;
+    }
+
+    std::string Identity::Key() const {
+        return std::format("{}:{}", pid, start);
+    }
+
+    std::optional<Identity> Self() {
+        if (!InSandbox()) {
+            const std::int64_t pid = getpid();
+            return Identity { .pid = pid, .start = ProcStart(pid).value_or(0) };
+        }
+        const std::optional<std::string> info = ReadFile(Paths().flatpakInfo);
+        const std::optional<std::string> instance = info ? KeyFileValue(*info, "Instance", "instance-id") : std::nullopt;
+        if (!instance) return std::nullopt;
+        const fs::path bwrapinfo = Paths().runtimeDir / ".flatpak" / *instance / "bwrapinfo.json";
+        // flatpak writes the file once bwrap reports the child's pid, which
+        // can be a moment after the application is already running.
+        for (std::int32_t attempt = 0; attempt < 20; ++attempt) {
+            if (const std::optional<std::string> text = ReadFile(bwrapinfo)) {
+                try {
+                    const Json parsed = Json::parse(*text);
+                    if (parsed.contains("child-pid") && parsed["child-pid"].is_number_integer()) {
+                        const std::optional<std::int64_t> start = ProcStart(1);
+                        if (start) return Identity { .pid = parsed["child-pid"].get<std::int64_t>(), .start = *start };
+                    }
+                } catch (const std::exception&) {
+                }
+            }
+            std::this_thread::sleep_for(std::chrono::milliseconds(100));
+        }
+        return std::nullopt;
+    }
+
+    std::int64_t PublishedPid() {
+        static const std::int64_t pid = [] {
+            if (const std::optional<Identity> self = Self()) return self->pid;
+            return static_cast<std::int64_t>(getpid());
+        }();
+        return pid;
+    }
+
+    fs::path LifetimeLockFile(const fs::path& dir, std::int64_t pid, std::int64_t start) {
+        return dir / std::format("{}-{}.lock", pid, start);
+    }
+
+    LifetimeLock::LifetimeLock(const fs::path& dir, const Identity& identity) : file(LifetimeLockFile(dir, identity.pid, identity.start)) {
+        fs::create_directories(dir);
+        if (ProbeLifetimeLock(dir, identity.pid, identity.start) == true) {
+            throw std::runtime_error(std::format("another driver already runs as {} (its lock {} is held): start each driver with its own `flatpak run`", identity.Key(), file.string()));
+        }
+        const fs::path temp = dir / std::format(".{}-{}.{}.tmp", identity.pid, identity.start, TokenHex(4));
+        fd = open(temp.c_str(), O_RDWR | O_CREAT | O_EXCL | O_CLOEXEC, 0644);
+        if (fd < 0) throw std::runtime_error(std::format("{}: {}", temp.string(), Dyn::ErrnoText(errno)));
+        if (flock(fd, LOCK_EX | LOCK_NB) != 0 || rename(temp.c_str(), file.c_str()) != 0) {
+            const std::int32_t failure = errno;
+            close(fd);
+            fd = -1;
+            unlink(temp.c_str());
+            throw std::runtime_error(std::format("{}: {}", file.string(), Dyn::ErrnoText(failure)));
+        }
+    }
+
+    LifetimeLock::~LifetimeLock() {
+        if (fd < 0) return;
+        // Unlinked while still held: a prober that opens the file after this
+        // finds none, which every rule reads as "this driver is gone or never
+        // took a lock" and the board's own leave() has already removed it.
+        unlink(file.c_str());
+        flock(fd, LOCK_UN);
+        close(fd);
+    }
+
+    const fs::path& LifetimeLock::File() const {
+        return file;
+    }
+
+    std::optional<bool> ProbeLifetimeLock(const fs::path& dir, std::int64_t pid, std::int64_t start) {
+        const fs::path file = LifetimeLockFile(dir, pid, start);
+        const std::int32_t fd = open(file.c_str(), O_RDONLY | O_CLOEXEC);
+        if (fd < 0) return std::nullopt;
+        // Shared: two probers looking at one dead driver both see it dead,
+        // where an exclusive probe would show each the other's lock.
+        const bool free = flock(fd, LOCK_SH | LOCK_NB) == 0;
+        if (free) flock(fd, LOCK_UN);
+        close(fd);
+        return !free;
+    }
+}
diff --git a/implementations/Crafter.Review-Process.cpp b/implementations/Crafter.Review-Process.cpp
new file mode 100644
index 0000000..bff1a1a
--- /dev/null
+++ b/implementations/Crafter.Review-Process.cpp
@@ -0,0 +1,473 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+module;
+#include <fcntl.h>
+#include <linux/close_range.h>
+#include <poll.h>
+#include <signal.h>
+#include <sys/prctl.h>
+#include <sys/syscall.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <cerrno>
+
+module Crafter.Review:Process_impl;
+import :Process;
+import :Platform;
+import :Text;
+import std;
+
+namespace fs = std::filesystem;
+
+namespace Crafter::Review::Process {
+    namespace {
+        // ---- the reaper's registry and the spawner's queue ------------------------
+        struct Entry {
+            std::optional<std::int32_t> status;
+        };
+        struct Request;
+        struct Shared {
+            std::mutex registryMutex;
+            std::condition_variable registryChanged;
+            std::map<pid_t, Entry> registry;
+            std::int32_t unreaped = 0;
+            std::mutex spawnMutex;
+            std::condition_variable spawnReady;
+            std::deque<Request*> spawnQueue;
+        };
+        // Never destroyed. The reaper and the spawner are detached threads
+        // that wait on these until the process ends, and a condition variable
+        // destroyed under a waiter blocks exit(): glibc's pthread_cond_destroy
+        // waits for its waiters, which here never come back.
+        Shared& State() {
+            static Shared* state = new Shared;
+            return *state;
+        }
+
+        std::int32_t ReturnCode(std::int32_t status) {
+            if (WIFEXITED(status)) return WEXITSTATUS(status);
+            if (WIFSIGNALED(status)) return -WTERMSIG(status);
+            return -1;
+        }
+
+        void ReaperLoop() {
+            while (true) {
+                std::int32_t status = 0;
+                const pid_t pid = waitpid(-1, &status, 0);
+                if (pid > 0) {
+                    const std::lock_guard lock(State().registryMutex);
+                    // A pid nobody registered is an orphan of one of our
+                    // children, reparented to us as the subreaper: reaped,
+                    // and that is all.
+                    if (const auto found = State().registry.find(pid); found != State().registry.end() && !found->second.status) {
+                        found->second.status = ReturnCode(status);
+                        --State().unreaped;
+                        State().registryChanged.notify_all();
+                    }
+                    continue;
+                }
+                if (errno == EINTR) continue;
+                // ECHILD: no child at all. Wait for Start() to register one;
+                // wake now and then for orphans reparented in the meantime.
+                std::unique_lock lock(State().registryMutex);
+                State().registryChanged.wait_for(lock, std::chrono::seconds(1), [] { return State().unreaped > 0; });
+            }
+        }
+
+        // ---- the spawner -------------------------------------------------------------
+        struct Request {
+            const Spec* spec;
+            std::promise<Child> result;
+        };
+        std::once_flag ThreadsStarted;
+
+        std::optional<std::string> FindOnPath(const std::string& name, const std::string& path) {
+            if (name.contains('/')) return name;
+            for (const std::string& dir : Dyn::Split(path, ":")) {
+                const std::string candidate = Dyn::JoinPath(dir.empty() ? std::string(".") : dir, name);
+                if (access(candidate.c_str(), X_OK) == 0) return candidate;
+            }
+            return std::nullopt;
+        }
+
+        std::int32_t OpenStream(const Stream& stream, bool reading, std::int32_t& ours) {
+            switch (stream.kind) {
+                case Stream::Kind::Inherit:
+                    return -1;
+                case Stream::Kind::Null:
+                    return open("/dev/null", (reading ? O_RDONLY : O_WRONLY) | O_CLOEXEC);
+                case Stream::Kind::File:
+                    return reading ? open(stream.file.c_str(), O_RDONLY | O_CLOEXEC)
+                                   : open(stream.file.c_str(), O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC, 0644);
+                case Stream::Kind::Pipe: {
+                    std::array<std::int32_t, 2> ends {};
+                    if (pipe2(ends.data(), O_CLOEXEC) != 0) return -2;
+                    ours = reading ? ends[1] : ends[0];
+                    return reading ? ends[0] : ends[1];
+                }
+            }
+            return -1;
+        }
+
+        Child DoSpawn(const Spec& spec) {
+            if (spec.argv.empty()) throw std::runtime_error("Start: no argv");
+            // Everything the child needs is made here: between fork and exec
+            // it may only make async-signal-safe calls.
+            std::vector<std::string> envStrings;
+            std::string pathVariable = "/usr/local/bin:/usr/bin:/bin";
+            if (spec.environment) {
+                for (const auto& [name, value] : *spec.environment) envStrings.push_back(std::format("{}={}", name, value));
+                if (const auto found = spec.environment->find("PATH"); found != spec.environment->end()) pathVariable = found->second;
+            } else {
+                for (char** entry = environ; entry != nullptr && *entry != nullptr; ++entry) envStrings.emplace_back(*entry);
+                if (const std::optional<std::string> path = Platform::GetEnv("PATH")) pathVariable = *path;
+            }
+            const std::optional<std::string> program = FindOnPath(spec.argv.front(), pathVariable);
+            if (!program) {
+                throw std::runtime_error(std::format("{}: {}", Dyn::ErrnoText(ENOENT), Dyn::Repr(spec.argv.front())));
+            }
+            std::vector<char*> argv;
+            for (const std::string& arg : spec.argv) argv.push_back(const_cast<char*>(arg.c_str()));
+            argv.push_back(nullptr);
+            std::vector<char*> envp;
+            for (const std::string& entry : envStrings) envp.push_back(const_cast<char*>(entry.c_str()));
+            envp.push_back(nullptr);
+            const std::string cwd = spec.cwd ? spec.cwd->string() : std::string();
+
+            Child child;
+            const std::int32_t in = OpenStream(spec.input, true, child.input);
+            const std::int32_t out = OpenStream(spec.output, false, child.output);
+            const std::int32_t err = OpenStream(spec.error, false, child.error);
+            auto closeAll = [&] {
+                for (const std::int32_t fd : { in, out, err, child.input, child.output, child.error }) {
+                    if (fd >= 0) close(fd);
+                }
+            };
+            if (in == -2 || out == -2 || err == -2 || (spec.input.kind != Stream::Kind::Inherit && in < 0)
+                || (spec.output.kind != Stream::Kind::Inherit && out < 0) || (spec.error.kind != Stream::Kind::Inherit && err < 0)) {
+                const std::int32_t failure = errno;
+                closeAll();
+                throw std::runtime_error(std::format("{}: {}", Dyn::ErrnoText(failure), Dyn::Repr(spec.argv.front())));
+            }
+            std::array<std::int32_t, 2> failurePipe {};
+            if (pipe2(failurePipe.data(), O_CLOEXEC) != 0) {
+                closeAll();
+                throw std::runtime_error(Dyn::ErrnoText(errno));
+            }
+            sigset_t empty;
+            sigemptyset(&empty);
+            const pid_t parent = getpid();
+            const std::int32_t deathSignal = spec.deathSignal;
+
+            std::unique_lock registry(State().registryMutex);
+            const pid_t pid = fork();
+            if (pid == 0) {
+                struct sigaction defaults {};
+                defaults.sa_handler = SIG_DFL;
+                sigaction(SIGPIPE, &defaults, nullptr);
+                sigaction(SIGXFSZ, &defaults, nullptr);
+                sigprocmask(SIG_SETMASK, &empty, nullptr);
+                prctl(PR_SET_PDEATHSIG, deathSignal);
+                if (getppid() != parent) _exit(127);
+                if (in >= 0) dup2(in, 0);
+                if (out >= 0) dup2(out, 1);
+                if (err >= 0) dup2(err, 2);
+                std::int32_t failure = 0;
+                if (!cwd.empty() && chdir(cwd.c_str()) != 0) {
+                    failure = errno;
+                } else {
+                    // Nothing but the three streams reaches the program: a lock
+                    // or a socket of the driver's held open by a child would
+                    // outlive the driver.
+                    syscall(SYS_close_range, 3U, ~0U, CLOSE_RANGE_CLOEXEC);
+                    execve(program->c_str(), argv.data(), envp.data());
+                    failure = errno;
+                }
+                (void)!write(failurePipe[1], &failure, sizeof(failure));
+                _exit(127);
+            }
+            if (pid < 0) {
+                const std::int32_t failure = errno;
+                registry.unlock();
+                closeAll();
+                close(failurePipe[0]);
+                close(failurePipe[1]);
+                throw std::runtime_error(std::format("fork: {}", Dyn::ErrnoText(failure)));
+            }
+            State().registry[pid] = {};
+            ++State().unreaped;
+            registry.unlock();
+            State().registryChanged.notify_all();
+
+            for (const std::int32_t fd : { in, out, err }) {
+                if (fd >= 0) close(fd);
+            }
+            close(failurePipe[1]);
+            std::int32_t failure = 0;
+            ssize_t got = 0;
+            do {
+                got = read(failurePipe[0], &failure, sizeof(failure));
+            } while (got < 0 && errno == EINTR);
+            close(failurePipe[0]);
+            if (got == static_cast<ssize_t>(sizeof(failure))) {
+                Wait(pid);
+                for (const std::int32_t fd : { child.input, child.output, child.error }) {
+                    if (fd >= 0) close(fd);
+                }
+                throw std::runtime_error(std::format("{}: {}", Dyn::ErrnoText(failure), Dyn::Repr(spec.argv.front())));
+            }
+            child.pid = pid;
+            return child;
+        }
+
+        void SpawnerLoop() {
+            while (true) {
+                Request* request = nullptr;
+                {
+                    std::unique_lock lock(State().spawnMutex);
+                    State().spawnReady.wait(lock, [] { return !State().spawnQueue.empty(); });
+                    request = State().spawnQueue.front();
+                    State().spawnQueue.pop_front();
+                }
+                try {
+                    request->result.set_value(DoSpawn(*request->spec));
+                } catch (...) {
+                    request->result.set_exception(std::current_exception());
+                }
+            }
+        }
+
+        void EnsureThreads() {
+            std::call_once(ThreadsStarted, [] {
+                std::thread(ReaperLoop).detach();
+                std::thread(SpawnerLoop).detach();
+            });
+        }
+    }
+
+    Child Start(const Spec& spec) {
+        EnsureThreads();
+        Request request { .spec = &spec };
+        std::future<Child> result = request.result.get_future();
+        {
+            const std::lock_guard lock(State().spawnMutex);
+            State().spawnQueue.push_back(&request);
+        }
+        State().spawnReady.notify_one();
+        return result.get();
+    }
+
+    std::optional<std::int32_t> Wait(std::int64_t pid, std::optional<std::chrono::milliseconds> timeout) {
+        std::unique_lock lock(State().registryMutex);
+        auto exited = [pid] {
+            const auto found = State().registry.find(static_cast<pid_t>(pid));
+            return found == State().registry.end() || found->second.status.has_value();
+        };
+        if (timeout) {
+            if (!State().registryChanged.wait_for(lock, *timeout, exited)) return std::nullopt;
+        } else {
+            State().registryChanged.wait(lock, exited);
+        }
+        const auto found = State().registry.find(static_cast<pid_t>(pid));
+        if (found == State().registry.end()) return -1;
+        const std::int32_t code = *found->second.status;
+        State().registry.erase(found);
+        return code;
+    }
+
+    void Signal(std::int64_t pid, std::int32_t signal) {
+        if (pid > 0) kill(static_cast<pid_t>(pid), signal);
+    }
+
+    void KillTree(std::int64_t root, bool includeRoot) {
+        std::set<std::int64_t> stopped;
+        for (std::int32_t round = 0; round < 50; ++round) {
+            const auto kids = Platform::PpidMap();
+            bool added = false;
+            for (const std::int64_t pid : Platform::Tree(root, kids)) {
+                if (pid == root && !includeRoot) continue;
+                if (pid <= 1) continue;
+                if (stopped.insert(pid).second) {
+                    kill(static_cast<pid_t>(pid), SIGSTOP);
+                    added = true;
+                }
+            }
+            if (!added) break;
+        }
+        for (const std::int64_t pid : stopped) kill(static_cast<pid_t>(pid), SIGKILL);
+    }
+
+    void KillDescendants() {
+        KillTree(getpid(), false);
+    }
+
+    Captured Run(const std::vector<std::string>& argv, const std::optional<fs::path>& cwd,
+                 std::optional<std::chrono::milliseconds> timeout, std::string_view input,
+                 const std::optional<std::map<std::string, std::string>>& environment) {
+        Spec spec { .argv = argv, .environment = environment, .cwd = cwd };
+        spec.input.kind = input.empty() ? Stream::Kind::Null : Stream::Kind::Pipe;
+        spec.output.kind = Stream::Kind::Pipe;
+        spec.error.kind = Stream::Kind::Pipe;
+        Child child = Start(spec);
+        Captured captured;
+        const auto deadline = timeout ? std::optional(std::chrono::steady_clock::now() + *timeout) : std::nullopt;
+        std::size_t sent = 0;
+        if (child.input >= 0) fcntl(child.input, F_SETFL, fcntl(child.input, F_GETFL) | O_NONBLOCK);
+        std::array<char, 65536> buffer {};
+        while (child.output >= 0 || child.error >= 0 || child.input >= 0) {
+            std::vector<pollfd> watched;
+            if (child.input >= 0) watched.push_back({ .fd = child.input, .events = POLLOUT, .revents = 0 });
+            if (child.output >= 0) watched.push_back({ .fd = child.output, .events = POLLIN, .revents = 0 });
+            if (child.error >= 0) watched.push_back({ .fd = child.error, .events = POLLIN, .revents = 0 });
+            std::int32_t waitMs = -1;
+            if (deadline) {
+                const auto left = std::chrono::duration_cast<std::chrono::milliseconds>(*deadline - std::chrono::steady_clock::now()).count();
+                if (left <= 0) {
+                    Signal(child.pid, SIGKILL);
+                    captured.timedOut = true;
+                    break;
+                }
+                waitMs = static_cast<std::int32_t>(std::min<std::int64_t>(left, 1000));
+            }
+            const std::int32_t ready = poll(watched.data(), watched.size(), waitMs);
+            if (ready < 0 && errno != EINTR) break;
+            for (const pollfd& entry : watched) {
+                if (entry.revents == 0) continue;
+                if (entry.fd == child.input) {
+                    if (entry.revents & (POLLERR | POLLHUP)) {
+                        close(child.input);
+                        child.input = -1;
+                        continue;
+                    }
+                    const ssize_t wrote = write(child.input, input.data() + sent, input.size() - sent);
+                    if (wrote > 0) sent += static_cast<std::size_t>(wrote);
+                    if (sent >= input.size() || (wrote < 0 && errno != EAGAIN && errno != EINTR)) {
+                        close(child.input);
+                        child.input = -1;
+                    }
+                    continue;
+                }
+                const ssize_t got = read(entry.fd, buffer.data(), buffer.size());
+                if (got > 0) {
+                    (entry.fd == child.output ? captured.out : captured.err).append(buffer.data(), static_cast<std::size_t>(got));
+                } else if (got == 0 || (errno != EINTR && errno != EAGAIN)) {
+                    close(entry.fd);
+                    (entry.fd == child.output ? child.output : child.error) = -1;
+                }
+            }
+        }
+        for (const std::int32_t fd : { child.input, child.output, child.error }) {
+            if (fd >= 0) close(fd);
+        }
+        captured.returnCode = Wait(child.pid).value_or(-1);
+        return captured;
+    }
+
+    void InstallSignalThread(std::function<void(std::int32_t)> onSignal) {
+        sigset_t set;
+        sigemptyset(&set);
+        sigaddset(&set, SIGINT);
+        sigaddset(&set, SIGTERM);
+        sigaddset(&set, SIGHUP);
+        pthread_sigmask(SIG_BLOCK, &set, nullptr);
+        struct sigaction ignore {};
+        ignore.sa_handler = SIG_IGN;
+        sigaction(SIGPIPE, &ignore, nullptr);
+        std::thread([set, onSignal = std::move(onSignal)] {
+            while (true) {
+                std::int32_t signal = 0;
+                if (sigwait(&set, &signal) == 0) onSignal(signal);
+            }
+        }).detach();
+    }
+
+    void BecomeSubreaper() {
+        prctl(PR_SET_CHILD_SUBREAPER, 1);
+    }
+
+    std::vector<std::string> Supervised(std::span<const std::string> argv) {
+        std::vector<std::string> out { Platform::SelfExe().string(), std::string(SuperviseVerb) };
+        out.insert(out.end(), argv.begin(), argv.end());
+        return out;
+    }
+
+    std::int32_t SupervisedReturnCode(std::int32_t supervisorCode) {
+        if (supervisorCode > 128 && supervisorCode <= 128 + 64) return -(supervisorCode - 128);
+        return supervisorCode;
+    }
+
+    std::int32_t SuperviseMain(std::span<const std::string_view> args) {
+        if (args.empty()) {
+            std::println(std::cerr, "{}: nothing to run", SuperviseVerb);
+            return 64;
+        }
+        prctl(PR_SET_CHILD_SUBREAPER, 1);
+        sigset_t watched;
+        sigemptyset(&watched);
+        sigaddset(&watched, SIGTERM);
+        sigaddset(&watched, SIGINT);
+        sigaddset(&watched, SIGHUP);
+        sigaddset(&watched, SIGCHLD);
+        sigprocmask(SIG_BLOCK, &watched, nullptr);
+        sigset_t empty;
+        sigemptyset(&empty);
+
+        std::vector<std::string> strings(args.begin(), args.end());
+        std::vector<char*> argv;
+        for (const std::string& arg : strings) argv.push_back(const_cast<char*>(arg.c_str()));
+        argv.push_back(nullptr);
+        const std::optional<std::string> program = FindOnPath(strings.front(), Platform::GetEnv("PATH").value_or("/usr/bin:/bin"));
+        if (!program) {
+            std::println(std::cerr, "{}: {}", Dyn::ErrnoText(ENOENT), Dyn::Repr(strings.front()));
+            return 127;
+        }
+        const pid_t self = getpid();
+        const pid_t target = fork();
+        if (target == 0) {
+            struct sigaction defaults {};
+            defaults.sa_handler = SIG_DFL;
+            sigaction(SIGPIPE, &defaults, nullptr);
+            sigaction(SIGXFSZ, &defaults, nullptr);
+            sigprocmask(SIG_SETMASK, &empty, nullptr);
+            prctl(PR_SET_PDEATHSIG, SIGKILL);
+            if (getppid() != self) _exit(127);
+            execv(program->c_str(), argv.data());
+            _exit(127);
+        }
+        if (target < 0) return 127;
+
+        std::optional<std::int32_t> status;
+        while (!status) {
+            siginfo_t info {};
+            const std::int32_t signal = sigwaitinfo(&watched, &info);
+            if (signal < 0) continue;
+            if (signal == SIGCHLD) {
+                while (true) {
+                    std::int32_t raw = 0;
+                    const pid_t pid = waitpid(-1, &raw, WNOHANG);
+                    if (pid <= 0) break;
+                    if (pid == target) status = raw;
+                }
+                continue;
+            }
+            // A timeout, an abort, or the driver gone: the whole tree goes,
+            // the target with it, and its status is what that left.
+            KillTree(self, false);
+            std::int32_t raw = 0;
+            while (waitpid(target, &raw, 0) < 0 && errno == EINTR) {}
+            status = raw;
+        }
+        // The target is gone; whatever it started goes too, and every orphan
+        // that was reparented to this subreaper is reaped.
+        KillTree(self, false);
+        while (true) {
+            const pid_t pid = waitpid(-1, nullptr, 0);
+            if (pid < 0 && errno == EINTR) continue;
+            if (pid < 0) break;
+        }
+        if (WIFEXITED(*status)) return WEXITSTATUS(*status);
+        if (WIFSIGNALED(*status)) return 128 + WTERMSIG(*status);
+        return 1;
+    }
+}
diff --git a/interfaces/Crafter.Review-Platform.cppm b/interfaces/Crafter.Review-Platform.cppm
new file mode 100644
index 0000000..ef55298
--- /dev/null
+++ b/interfaces/Crafter.Review-Platform.cppm
@@ -0,0 +1,173 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+export module Crafter.Review:Platform;
+import std;
+import :Json;
+
+// The machine as the driver sees it: whether it runs inside the Flatpak's
+// sandbox, its environment, the files it shares with other drivers and how
+// they are locked, /proc, and the identity it is known by on the seat board.
+//
+// The sandbox is decided once, by Init(). Everything that reads /proc or the
+// sandbox's instance files goes through Paths(), which a test points at a
+// fake tree: flatpak-builder runs the suite inside `flatpak build`, where
+// /.flatpak-info exists, and a test that read the real files would test the
+// build sandbox instead of the code.
+export namespace Crafter::Review::Platform {
+    enum class Mode {
+        Native,
+        // Inside a Flatpak: its own PID namespace, XDG directories under
+        // ~/.var/app, a private /tmp unless the manifest shares the host's.
+        Sandbox,
+    };
+
+    struct SystemPaths {
+        std::filesystem::path proc = "/proc";
+        std::filesystem::path flatpakInfo = "/.flatpak-info";
+        // $XDG_RUNTIME_DIR, where the instance's bwrapinfo.json is mounted:
+        // /run/user/<uid>/.flatpak/<instance-id>/bwrapinfo.json.
+        std::filesystem::path runtimeDir;
+    };
+
+    // Decides the mode from /.flatpak-info and, in the sandbox, restores the
+    // host's XDG directories (RestoreHostXdg) before any thread starts.
+    void Init();
+    Mode CurrentMode();
+    bool InSandbox();
+    // For tests: pin the mode and the paths the identity code reads.
+    void SetMode(Mode mode);
+    const SystemPaths& Paths();
+    void SetPaths(SystemPaths paths);
+
+    // ---- environment -----------------------------------------------------------
+    std::optional<std::string> GetEnv(std::string_view name);
+    // The process environment.
+    std::map<std::string, std::string> Environment();
+    // $HOME, else the password database's.
+    std::string Home();
+    // Flatpak points XDG_{CONFIG,CACHE,DATA,STATE}_HOME at ~/.var/app/<id>/…;
+    // they go back to the host's (HOST_XDG_*, else the XDG defaults under
+    // $HOME), so git reads ~/.config/git/config, Playwright finds
+    // ~/.cache/ms-playwright, and claude and npx behave as on the host.
+    void RestoreHostXdg();
+    // The absolute path of the running binary (/proc/self/exe).
+    std::filesystem::path SelfExe();
+
+    // ---- time ----------------------------------------------------------------------
+    // time.time()
+    double Now();
+    // time.strftime(format) in local time.
+    std::string LocalTime(std::string_view format);
+    // time.strftime(format, time.localtime(epochSeconds))
+    std::string LocalTime(std::string_view format, double epochSeconds);
+    // date.today().isoformat()
+    std::string TodayIso();
+
+    // ---- files ------------------------------------------------------------------------
+    // The file's bytes, or nullopt when it cannot be read.
+    std::optional<std::string> ReadFile(const std::filesystem::path& path);
+    // `content` into `<path>.<pid>.tmp`, created with `mode` (the umask
+    // applies), then renamed over `path`: a reader never sees half a file, and
+    // two writers never share a temp name. The pid is PublishedPid(), which in
+    // the sandbox is a host pid, where every driver would otherwise be pid 2.
+    // Directories are created as needed.
+    void WriteAtomic(const std::filesystem::path& path, std::string_view content, std::uint32_t mode = 0644);
+
+    // An exclusive flock(2) on `lockPath`, held while the object lives. flock,
+    // not fcntl or OFD locks: every driver on the machine takes flock on the
+    // same side files, and on Linux the two kinds do not exclude each other.
+    class FileLock {
+    public:
+        explicit FileLock(const std::filesystem::path& lockPath);
+        ~FileLock();
+        FileLock(const FileLock&) = delete;
+        FileLock& operator=(const FileLock&) = delete;
+        // False when the lock file could not be opened or locked; the caller
+        // carries on best-effort.
+        bool Held() const;
+        // Why not, when it is not held.
+        const std::string& Problem() const;
+
+    private:
+        std::int32_t fd = -1;
+        std::string problem;
+    };
+
+    // The one shape every file shared between drivers is written in (the
+    // board, the ledger, memory.json, unit-state.json): lock `<path>.lock`,
+    // re-read the file, let `change` edit it in place — a false return leaves
+    // the file as it is — write it back atomically with `mode`, release.
+    // A file that does not parse as a JSON object hands `change` a copy of
+    // `fallback`. Without the lock the write is still made, best-effort;
+    // `warn` hears about that and about a write that failed. Returns what
+    // `change` saw.
+    Json LockedUpdate(const std::filesystem::path& path, const std::function<bool(Json&)>& change, const Json& fallback,
+                      std::uint32_t mode, const std::function<void(const std::string&)>& warn);
+
+    // ---- /proc ----------------------------------------------------------------------------
+    // The start time of a live process, in clock ticks since boot (field 22
+    // of /proc/<pid>/stat, read after the last ')' because the command name
+    // may hold spaces and parentheses); nullopt for no such process or a
+    // zombie. Under Paths().proc.
+    std::optional<std::int64_t> ProcStart(std::int64_t pid);
+    // The first 8 characters of /proc/sys/kernel/random/boot_id, "" when
+    // unreadable.
+    std::string BootId();
+    // /proc/meminfo as {field: MB}; a field without a kB unit keeps its value.
+    std::map<std::string, std::int64_t> MemInfo(const std::filesystem::path& path = "/proc/meminfo");
+    // {ppid: [pids]} of every process now, from one read of each
+    // /proc/<pid>/stat. Always the real /proc: a process tree is this
+    // driver's own business.
+    std::map<std::int64_t, std::vector<std::int64_t>> PpidMap();
+    // The pid and every descendant in `kids`.
+    std::vector<std::int64_t> Tree(std::int64_t pid, const std::map<std::int64_t, std::vector<std::int64_t>>& kids);
+    // The summed Pss of the processes, in MB; one that is gone counts nothing.
+    double PssMb(std::span<const std::int64_t> pids);
+
+    // ---- identity -------------------------------------------------------------------------
+    // How the other drivers on this machine know a driver: `<pid>:<start>`.
+    struct Identity {
+        std::int64_t pid = 0;
+        std::int64_t start = 0;
+        std::string Key() const;
+    };
+    // This process as every driver on the machine can check it. Natively its
+    // own pid and start time. In the sandbox its pid means nothing outside
+    // (every Flatpak driver is pid 2), so it is the instance's: the host pid
+    // of the sandbox's init (`child-pid` in bwrapinfo.json) and the start time
+    // of /proc/1 inside, which is the same process — a driver outside that
+    // checks the pid then sees it alive exactly as long as the sandbox runs.
+    // nullopt in a sandbox without bwrapinfo.json (`flatpak build`), after
+    // waiting up to 2 s for flatpak to write it.
+    std::optional<Identity> Self();
+    // The pid to put in temp names and lock information: Self()'s, else
+    // getpid().
+    std::int64_t PublishedPid();
+
+    // The lifetime lock of a driver: an exclusive flock on
+    // `<dir>/<pid>-<start>.lock`, held as long as the driver lives. It is
+    // created under a temporary name, locked, then renamed into place, so a
+    // prober never finds the file unlocked while its owner lives. Another
+    // driver tells a live one from a dead one by it across sandboxes, where
+    // pids are not comparable.
+    class LifetimeLock {
+    public:
+        // Throws std::runtime_error when the name is already held by a live
+        // driver: a second full run in one sandbox instance.
+        LifetimeLock(const std::filesystem::path& dir, const Identity& identity);
+        ~LifetimeLock();
+        LifetimeLock(const LifetimeLock&) = delete;
+        LifetimeLock& operator=(const LifetimeLock&) = delete;
+        const std::filesystem::path& File() const;
+
+    private:
+        std::int32_t fd = -1;
+        std::filesystem::path file;
+    };
+    std::filesystem::path LifetimeLockFile(const std::filesystem::path& dir, std::int64_t pid, std::int64_t start);
+    // true: a live driver holds the lock; false: the file is there and nobody
+    // holds it (its driver died); nullopt: there is no lock file (a driver
+    // that takes none).
+    std::optional<bool> ProbeLifetimeLock(const std::filesystem::path& dir, std::int64_t pid, std::int64_t start);
+}
diff --git a/interfaces/Crafter.Review-Process.cppm b/interfaces/Crafter.Review-Process.cppm
new file mode 100644
index 0000000..28a5c73
--- /dev/null
+++ b/interfaces/Crafter.Review-Process.cppm
@@ -0,0 +1,113 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+export module Crafter.Review:Process;
+import std;
+
+// Child processes, and making sure none outlives what it was started for.
+//
+// SIGKILL to the claude process alone would leave the Playwright server and
+// Chromium under it running after a timeout, and every pass running after a
+// SIGKILLed driver. So:
+//  - every child is started from one long-lived spawner thread, because
+//    PR_SET_PDEATHSIG fires when the *thread* that forked the child ends;
+//  - the driver is a child subreaper, so whatever its children leave
+//    behind becomes its own child rather than init's, and one reaper thread
+//    owns waitpid(-1) for all of them;
+//  - a claude pass runs under a supervisor — this binary again, as
+//    `crafter-review __supervise <argv>` — that is the subreaper of the
+//    pass's whole tree. Playwright starts Chromium detached, as the leader of
+//    a process group of its own, so killing a process group would miss it; a
+//    tree walk does not. The supervisor kills the tree when claude exits, on
+//    SIGTERM (a timeout or an abort), and when the driver dies however it
+//    dies, through PR_SET_PDEATHSIG.
+//
+// Every subprocess goes through Start(); popen() and system() are not used.
+export namespace Crafter::Review::Process {
+    // Where a child's standard stream goes.
+    struct Stream {
+        enum class Kind {
+            Inherit,
+            Null,
+            // Opened for reading (stdin) or created and truncated (stdout,
+            // stderr).
+            File,
+            // A pipe whose other end Start() returns.
+            Pipe,
+        };
+        Kind kind = Kind::Inherit;
+        std::filesystem::path file;
+    };
+
+    struct Spec {
+        // argv[0] is looked up on the child's PATH when it has no '/'.
+        std::vector<std::string> argv;
+        // nullopt: this process's environment.
+        std::optional<std::map<std::string, std::string>> environment;
+        std::optional<std::filesystem::path> cwd;
+        Stream input;
+        Stream output;
+        Stream error;
+        // Sent to the child when this process ends (9, SIGKILL; a supervisor
+        // is started with 15, SIGTERM, so it can take its tree down first).
+        std::int32_t deathSignal = 9;
+    };
+
+    struct Child {
+        std::int64_t pid = 0;
+        // Our ends of the pipes the spec asked for, -1 otherwise; the caller
+        // closes them.
+        std::int32_t input = -1;
+        std::int32_t output = -1;
+        std::int32_t error = -1;
+    };
+
+    // Throws std::runtime_error when the child cannot be started, with the
+    // errno text: "[Errno 2] No such file or directory: 'claude'".
+    Child Start(const Spec& spec);
+    // The return code once the child has exited — its exit status, or minus
+    // the signal that killed it — or nullopt when `timeout` ran out first, the
+    // child still running.
+    std::optional<std::int32_t> Wait(std::int64_t pid, std::optional<std::chrono::milliseconds> timeout = std::nullopt);
+    void Signal(std::int64_t pid, std::int32_t signal);
+    // Every descendant of `root` (and root itself with includeRoot) is
+    // stopped with SIGSTOP, the tree is scanned again until no new process
+    // appears — a stopped process cannot fork — and then all are killed.
+    void KillTree(std::int64_t root, bool includeRoot);
+    // KillTree of this process, at exit: never pid 1, never a process this
+    // process did not start, such as a debug shell in the same sandbox.
+    void KillDescendants();
+
+    // A command run to its end with both outputs captured: stdin from
+    // `input`, or /dev/null; both outputs read as they come, so neither can
+    // fill its pipe and stall the child. With a timeout the child is killed
+    // when it runs out, and timedOut is set.
+    struct Captured {
+        std::int32_t returnCode = -1;
+        std::string out;
+        std::string err;
+        bool timedOut = false;
+    };
+    Captured Run(const std::vector<std::string>& argv, const std::optional<std::filesystem::path>& cwd = std::nullopt,
+                 std::optional<std::chrono::milliseconds> timeout = std::nullopt, std::string_view input = {},
+                 const std::optional<std::map<std::string, std::string>>& environment = std::nullopt);
+
+    // Main() calls both first, before any other thread starts. SIGINT,
+    // SIGTERM and SIGHUP are blocked in every thread and handed to `onSignal`
+    // on a thread of their own; SIGPIPE is ignored, so a helper that goes away
+    // mid-call cannot end a run that has been going for hours. Children get
+    // both back at their defaults.
+    void InstallSignalThread(std::function<void(std::int32_t)> onSignal);
+    void BecomeSubreaper();
+
+    constexpr std::string_view SuperviseVerb = "__supervise";
+    // The argv that runs `argv` under a supervisor: this binary, the verb,
+    // then `argv`.
+    std::vector<std::string> Supervised(std::span<const std::string> argv);
+    // The supervisor itself, given everything after the verb. Exits with the
+    // target's status, or 128 + the signal that killed it.
+    std::int32_t SuperviseMain(std::span<const std::string_view> argv);
+    // A supervisor's exit code as the target's return code: 128 + n up to
+    // 128 + 64 is "killed by signal n", -n.
+    std::int32_t SupervisedReturnCode(std::int32_t supervisorCode);
+}
diff --git a/interfaces/Crafter.Review.cppm b/interfaces/Crafter.Review.cppm
index 652c381..fe0a7f2 100644
--- a/interfaces/Crafter.Review.cppm
+++ b/interfaces/Crafter.Review.cppm
@@ -7,5 +7,7 @@ export import :Unicode;
 export import :Json;
 export import :Text;
 export import :Crypto;
+export import :Platform;
+export import :Process;
 export import :Core;
 export import :Driver;
diff --git a/project.cpp b/project.cpp
index 4ea4b5d..0b6ed82 100644
--- a/project.cpp
+++ b/project.cpp
@@ -13,27 +13,33 @@ using namespace Crafter;
 namespace {
     // The umbrella, then the partitions in import order: each after the ones
     // it imports, which is also the order the module map in AGENTS.md uses.
-    constexpr std::array<std::string_view, 7> Interfaces = {
+    constexpr std::array<std::string_view, 9> Interfaces = {
         "interfaces/Crafter.Review",
         "interfaces/Crafter.Review-Unicode",
         "interfaces/Crafter.Review-Json",
         "interfaces/Crafter.Review-Text",
         "interfaces/Crafter.Review-Crypto",
+        "interfaces/Crafter.Review-Platform",
+        "interfaces/Crafter.Review-Process",
         "interfaces/Crafter.Review-Core",
         "interfaces/Crafter.Review-Driver",
     };
-    constexpr std::array<std::string_view, 4> Implementations = {
+    constexpr std::array<std::string_view, 6> Implementations = {
         "implementations/Crafter.Review-Text",
         "implementations/Crafter.Review-Crypto",
+        "implementations/Crafter.Review-Platform",
+        "implementations/Crafter.Review-Process",
         "implementations/Crafter.Review-Core",
         "implementations/Crafter.Review-Driver",
     };
 
     // One per directory under tests/, each an ordinary main() that links the
     // library: exit 0 passes, nonzero fails, 77 skips.
-    constexpr std::array<std::string_view, 2> Tests = {
+    constexpr std::array<std::string_view, 4> Tests = {
         "Crypto",
         "Text",
+        "Platform",
+        "Process",
     };
 
     // The version the binary reports: CI sets it from the tag, a checkout
diff --git a/tests/Platform/main.cpp b/tests/Platform/main.cpp
new file mode 100644
index 0000000..bb0b9af
--- /dev/null
+++ b/tests/Platform/main.cpp
@@ -0,0 +1,219 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// The machine as the driver sees it.
+//
+// - Shared files: an atomic write never leaves half a file and gives the
+//   temp file the mode asked for; LockedUpdate re-reads under the lock, keeps
+//   keys it does not know, falls back on a damaged file, and serialises
+//   concurrent writers — including another process holding flock(2) on the
+//   same side file, as every driver on the machine does.
+// - /proc: the start time read after the last ')' (a command name may hold
+//   spaces and parentheses), a zombie reading as gone, MemInfo in MB.
+// - Identity: natively a driver's own pid and start time; in the sandbox the
+//   instance's host pid from bwrapinfo.json with the start time of /proc/1.
+//   On a fake /proc and a fake instance directory, because the suite itself
+//   runs inside `flatpak build` in the Flatpak.
+// - The lifetime lock: held while its driver lives, refused to a second
+//   driver under the same name, gone when it ends, and a stale one reads as
+//   dead.
+// - The host XDG directories coming back in the sandbox.
+
+#include <sys/stat.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+import std;
+import Crafter.Review;
+namespace fs = std::filesystem;
+using namespace Crafter::Review;
+using namespace std::chrono_literals;
+
+namespace {
+    std::int32_t Failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++Failures;
+        }
+    }
+
+    void Write(const fs::path& path, std::string_view text) {
+        fs::create_directories(path.parent_path());
+        std::ofstream(path, std::ios::binary) << text;
+    }
+
+    // A /proc/<pid>/stat whose command name is designed to trip a naive split.
+    std::string Stat(std::int64_t pid, char state, std::int64_t start) {
+        std::string fields = std::format("{} (we (ird) name) {} 1", pid, state);
+        for (std::int32_t field = 5; field <= 21; ++field) fields += " 0";
+        return std::format("{} {} 0 0\n", fields, start);
+    }
+}
+
+int main() {
+    const fs::path dir = fs::temp_directory_path() / std::format("crafter-review-platform-{}", getpid());
+    fs::remove_all(dir);
+    fs::create_directories(dir);
+    Platform::SetMode(Platform::Mode::Native);
+
+    {
+        const mode_t previous = umask(022);
+        const fs::path file = dir / "atomic.json";
+        Platform::WriteAtomic(file, "{\"a\": 1}", 0600);
+        struct stat info {};
+        stat(file.c_str(), &info);
+        Check(Platform::ReadFile(file) == "{\"a\": 1}", "the atomic write lands");
+        Check((info.st_mode & 0777) == 0600, std::format("with the mode asked for ({:o})", info.st_mode & 0777));
+        Check(!fs::exists(std::format("{}.{}.tmp", file.string(), Platform::PublishedPid())), "and leaves no temp file");
+        umask(previous);
+    }
+
+    {
+        const fs::path file = dir / "board.json";
+        Write(file, "{\"version\": 1, \"cost7d\": 3.5, \"seats\": {}}");
+        const Json seen = Platform::LockedUpdate(file, [](Json& data) {
+            data["seats"]["local"] = { { "id", "local" } };
+            return true;
+        }, Json::object(), 0600, {});
+        const Json back = Json::parse(*Platform::ReadFile(file));
+        Check(back["cost7d"] == 3.5 && back["seats"]["local"]["id"] == "local", "a key this driver does not know survives the rewrite");
+        Check(*Platform::ReadFile(file) == DumpFile(back), "written in DumpFile's layout");
+        Check(seen == back, "LockedUpdate returns what the change saw");
+
+        Write(file, "{not json");
+        const Json fallback = { { "version", 1 } };
+        Platform::LockedUpdate(file, [](Json& data) {
+            return data.contains("version");
+        }, fallback, 0600, {});
+        Check(Json::parse(*Platform::ReadFile(file)) == fallback, "a damaged file is replaced by the fallback the change kept");
+
+        const std::string before = *Platform::ReadFile(file);
+        Platform::LockedUpdate(file, [](Json& data) {
+            data["ignored"] = true;
+            return false;
+        }, Json::object(), 0600, {});
+        Check(*Platform::ReadFile(file) == before, "a change that returns false leaves the file alone");
+    }
+
+    {
+        // Two writers in one process: flock is per open file description, so
+        // two FileLocks exclude each other just as two processes would.
+        const fs::path file = dir / "counter.json";
+        Write(file, "{\"n\": 0}");
+        auto bump = [&file] {
+            for (std::int32_t i = 0; i < 100; ++i) {
+                Platform::LockedUpdate(file, [](Json& data) {
+                    data["n"] = data["n"].get<std::int64_t>() + 1;
+                    return true;
+                }, Json::object(), 0600, {});
+            }
+        };
+        std::thread a(bump);
+        std::thread b(bump);
+        a.join();
+        b.join();
+        Check(Json::parse(*Platform::ReadFile(file))["n"] == 200, "concurrent read-modify-writes lose nothing");
+    }
+
+    {
+        // Another process holds flock on the side file for a moment (flock(1),
+        // util-linux): the lock here must wait for it, as every driver on the
+        // machine shares these files.
+        const fs::path file = dir / "shared.json";
+        const fs::path lock = std::format("{}.lock", file.string());
+        const fs::path ready = dir / "other-holds";
+        try {
+            const Process::Child other = Process::Start({ .argv = { "flock", lock.string(), "sh", "-c", "touch \"$1\" && sleep 0.4", "sh", ready.string() } });
+            for (std::int32_t i = 0; i < 100 && !fs::exists(ready); ++i) std::this_thread::sleep_for(20ms);
+            const auto start = std::chrono::steady_clock::now();
+            {
+                const Platform::FileLock ours(lock);
+                Check(ours.Held(), "the lock is taken once the other process lets go");
+            }
+            const auto waited = std::chrono::steady_clock::now() - start;
+            Process::Wait(other.pid);
+            Check(waited >= 200ms, std::format("and not before: flock(2) excludes the other process's flock ({} ms)", std::chrono::duration_cast<std::chrono::milliseconds>(waited).count()));
+        } catch (const std::runtime_error&) {
+            std::println("SKIP  no flock here to hold the other end of the lock");
+        }
+    }
+
+    {
+        const fs::path proc = dir / "proc";
+        Write(proc / "4242" / "stat", Stat(4242, 'S', 987654));
+        Write(proc / "4243" / "stat", Stat(4243, 'Z', 5));
+        Write(proc / "sys" / "kernel" / "random" / "boot_id", "0123456789abcdef-the-rest\n");
+        Write(dir / "meminfo", "MemTotal:       65536000 kB\nMemAvailable:   32768000 kB\nHugePages_Total:       0\n");
+        Platform::SetPaths({ .proc = proc, .flatpakInfo = dir / "no-flatpak-info", .runtimeDir = dir / "run" });
+        Check(Platform::ProcStart(4242) == 987654, "the start time is field 22, read after the last ')'");
+        Check(!Platform::ProcStart(4243).has_value(), "a zombie reads as gone");
+        Check(!Platform::ProcStart(4244).has_value(), "so does a pid that is not there");
+        Check(Platform::BootId() == "01234567", "boot_id's first eight characters");
+        const auto memory = Platform::MemInfo(dir / "meminfo");
+        Check(memory.at("MemTotal") == 64000 && memory.at("MemAvailable") == 32000 && memory.at("HugePages_Total") == 0, "meminfo in MB, unitless fields as they are");
+
+        Write(proc / std::to_string(getpid()) / "stat", Stat(getpid(), 'R', 111));
+        Platform::SetMode(Platform::Mode::Native);
+        const std::optional<Platform::Identity> native = Platform::Self();
+        Check(native && native->pid == getpid() && native->start == 111 && native->Key() == std::format("{}:111", getpid()), "natively a driver is its own pid and start time");
+
+        Write(dir / "flatpak-info", "[Application]\nname=net.catcrafts.CrafterReview\n\n[Instance]\ninstance-id=2509661707\napp-path=/x\n");
+        Write(dir / "run" / ".flatpak" / "2509661707" / "bwrapinfo.json", "{\n    \"child-pid\": 416559,\n    \"pid-namespace\": 4026536671\n}\n");
+        Write(proc / "1" / "stat", Stat(1, 'S', 841601));
+        Platform::SetPaths({ .proc = proc, .flatpakInfo = dir / "flatpak-info", .runtimeDir = dir / "run" });
+        Platform::SetMode(Platform::Mode::Sandbox);
+        const std::optional<Platform::Identity> sandboxed = Platform::Self();
+        Check(sandboxed && sandboxed->Key() == "416559:841601", "in the sandbox: the instance's host pid and /proc/1's start time");
+
+        fs::remove(dir / "run" / ".flatpak" / "2509661707" / "bwrapinfo.json");
+        Check(!Platform::Self().has_value(), "no bwrapinfo.json (flatpak build): no identity a host driver could check");
+        Platform::SetMode(Platform::Mode::Native);
+    }
+
+    {
+        const fs::path drivers = dir / "drivers";
+        const Platform::Identity me { .pid = 416559, .start = 841601 };
+        {
+            const Platform::LifetimeLock lock(drivers, me);
+            Check(fs::exists(drivers / "416559-841601.lock"), "the lifetime lock is named <pid>-<start>.lock");
+            Check(Platform::ProbeLifetimeLock(drivers, me.pid, me.start) == true, "a probe finds it held");
+            bool refused = false;
+            try {
+                const Platform::LifetimeLock second(drivers, me);
+            } catch (const std::runtime_error&) {
+                refused = true;
+            }
+            Check(refused, "a second driver under the same name is refused");
+            const std::vector<fs::directory_entry> entries(fs::directory_iterator(drivers), {});
+            Check(entries.size() == 1, "and leaves no temp file behind");
+        }
+        Check(!Platform::ProbeLifetimeLock(drivers, me.pid, me.start).has_value(), "a driver that ended leaves no lock file");
+        Write(Platform::LifetimeLockFile(drivers, 7, 8), "");
+        Check(Platform::ProbeLifetimeLock(drivers, 7, 8) == false, "a lock file nobody holds is a dead driver's");
+        Check(!Platform::ProbeLifetimeLock(drivers, 9, 9).has_value(), "no lock file: a driver that takes none");
+    }
+
+    {
+        const std::string home = Platform::Home();
+        setenv("XDG_CONFIG_HOME", "/home/x/.var/app/net.catcrafts.CrafterReview/config", 1);
+        setenv("XDG_CACHE_HOME", "/home/x/.var/app/net.catcrafts.CrafterReview/cache", 1);
+        setenv("HOST_XDG_CONFIG_HOME", "/elsewhere/config", 1);
+        unsetenv("HOST_XDG_CACHE_HOME");
+        Platform::RestoreHostXdg();
+        Check(Platform::GetEnv("XDG_CONFIG_HOME") == "/elsewhere/config", "a host XDG directory the host set comes back");
+        Check(Platform::GetEnv("XDG_CACHE_HOME") == Dyn::JoinPath(home, ".cache"), "one it left unset is the XDG default under $HOME");
+        Check(Platform::GetEnv("XDG_STATE_HOME") == Dyn::JoinPath(home, ".local/state"), "state too");
+    }
+
+    {
+        const std::string today = Platform::TodayIso();
+        Check(today.size() == 10 && today[4] == '-' && today[7] == '-', "date.today().isoformat()");
+        Check(Platform::LocalTime("%H:%M:%S", 0).size() == 8, "strftime in local time");
+        Check(Platform::SelfExe().is_absolute(), "the running binary's path");
+    }
+
+    fs::remove_all(dir);
+    return Failures == 0 ? 0 : 1;
+}
diff --git a/tests/Process/main.cpp b/tests/Process/main.cpp
new file mode 100644
index 0000000..efe9252
--- /dev/null
+++ b/tests/Process/main.cpp
@@ -0,0 +1,207 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Child processes and their lifetime.
+//
+// SIGKILL to the claude process alone would leave the Playwright server and
+// Chromium under it running after a timeout, and every pass running after a
+// SIGKILLed driver, so a pass runs under a supervisor that is the subreaper
+// of its tree. This test starts a target that leaves a detached grandchild —
+// `setsid sleep`, a process group of its own, the way Playwright starts
+// Chromium — and checks the grandchild is gone when the target exits, when
+// the supervisor is told to stop, and when the driver above it is SIGKILLed.
+//
+// Also: return codes (minus the signal for a killed child), a missing
+// program reported with its errno text, captured output from both streams
+// without a deadlock, a timeout, nothing but the three streams reaching the
+// child, and the child getting SIGPIPE back at its default with an empty
+// signal mask while the driver ignores SIGPIPE.
+//
+// The test binary plays the supervisor (`__supervise`) and the driver
+// (`--as-driver`) itself, by re-running its own executable.
+
+#include <fcntl.h>
+#include <signal.h>
+#include <unistd.h>
+#include <cerrno>
+
+import std;
+import Crafter.Review;
+namespace fs = std::filesystem;
+using namespace Crafter::Review;
+using namespace std::chrono_literals;
+
+namespace {
+    std::int32_t Failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++Failures;
+        }
+    }
+
+    fs::path Stage() {
+        const fs::path dir = fs::temp_directory_path() / std::format("crafter-review-process-{}", getpid());
+        fs::remove_all(dir);
+        fs::create_directories(dir);
+        return dir;
+    }
+
+    bool Alive(std::int64_t pid) {
+        return pid > 0 && kill(static_cast<pid_t>(pid), 0) == 0;
+    }
+
+    // The pid a target wrote to `file`, waited for up to 5 s.
+    std::int64_t PidFrom(const fs::path& file) {
+        for (std::int32_t i = 0; i < 100; ++i) {
+            if (const std::optional<std::string> text = Platform::ReadFile(file)) {
+                if (const std::optional<std::int64_t> pid = Dyn::ParseInt(*text); pid && *pid > 0) return *pid;
+            }
+            std::this_thread::sleep_for(50ms);
+        }
+        return 0;
+    }
+
+    bool GoneWithin(std::int64_t pid, std::chrono::milliseconds limit) {
+        const auto deadline = std::chrono::steady_clock::now() + limit;
+        while (std::chrono::steady_clock::now() < deadline) {
+            if (!Alive(pid)) return true;
+            std::this_thread::sleep_for(20ms);
+        }
+        return !Alive(pid);
+    }
+
+    // `sh -c '<script>'` under a supervisor, started the way the pool starts
+    // a pass: SIGTERM as its death signal.
+    Process::Child StartSupervised(const std::string& script) {
+        const std::vector<std::string> target { "sh", "-c", script };
+        Process::Spec spec { .argv = Process::Supervised(target) };
+        spec.deathSignal = SIGTERM;
+        return Process::Start(spec);
+    }
+
+    // The driver this test SIGKILLs: it starts a supervised target that
+    // leaves a detached grandchild, then waits to be killed.
+    std::int32_t AsDriver(std::string_view pidFile) {
+        Process::BecomeSubreaper();
+        StartSupervised(std::format("setsid sleep 300 & echo $! > {}; wait", pidFile));
+        std::this_thread::sleep_for(60s);
+        return 0;
+    }
+
+    std::uint64_t HexMask(const std::string& status, std::string_view field) {
+        for (const std::string& line : Dyn::SplitLines(status)) {
+            if (!line.starts_with(field)) continue;
+            return std::stoull(Dyn::Strip(line.substr(field.size())), nullptr, 16);
+        }
+        return ~0ULL;
+    }
+}
+
+int main(int argc, char** argv) {
+    const std::vector<std::string_view> args(argv, argv + argc);
+    if (args.size() > 1 && args[1] == Process::SuperviseVerb) return Process::SuperviseMain(std::span(args).subspan(2));
+    if (args.size() > 2 && args[1] == "--as-driver") return AsDriver(args[2]);
+
+    Process::InstallSignalThread([](std::int32_t) {});
+    const fs::path dir = Stage();
+
+    {
+        const Process::Child child = Process::Start({ .argv = { "sh", "-c", "exit 3" } });
+        Check(Process::Wait(child.pid) == 3, "an exit status comes back as the return code");
+        const Process::Child killed = Process::Start({ .argv = { "sh", "-c", "kill -TERM $$" } });
+        Check(Process::Wait(killed.pid) == -15, "a child killed by a signal returns minus the signal, as Popen does");
+    }
+
+    {
+        std::string message;
+        try {
+            Process::Start({ .argv = { "no-such-program-for-crafter-review" } });
+        } catch (const std::runtime_error& e) {
+            message = e.what();
+        }
+        Check(message == "[Errno 2] No such file or directory: 'no-such-program-for-crafter-review'",
+              std::format("a missing program fails the way Popen fails ({})", message));
+    }
+
+    {
+        const Process::Captured both = Process::Run({ "sh", "-c", "echo out; echo err >&2; exit 4" });
+        Check(both.out == "out\n" && both.err == "err\n" && both.returnCode == 4, "both streams and the return code are captured");
+        const Process::Captured fed = Process::Run({ "cat" }, std::nullopt, std::nullopt, "fed through stdin");
+        Check(fed.out == "fed through stdin", "input reaches the child's stdin");
+        const Process::Captured large = Process::Run({ "sh", "-c", "head -c 300000 /dev/zero; head -c 300000 /dev/zero >&2" });
+        Check(large.out.size() == 300000 && large.err.size() == 300000, "large output on both streams does not stall the child");
+        const auto start = std::chrono::steady_clock::now();
+        const Process::Captured slow = Process::Run({ "sleep", "5" }, std::nullopt, 200ms);
+        Check(slow.timedOut && std::chrono::steady_clock::now() - start < 3s, "a timeout kills the child and comes back promptly");
+        const Process::Captured where = Process::Run({ "pwd" }, dir);
+        Check(Dyn::Strip(where.out) == fs::canonical(dir).string(), "the child runs in the directory asked for");
+        const Process::Captured env = Process::Run({ "sh", "-c", "echo $ONLY_HERE" }, std::nullopt, std::nullopt, {},
+                                                   std::map<std::string, std::string> { { "ONLY_HERE", "yes" }, { "PATH", "/usr/bin:/bin" } });
+        Check(Dyn::Strip(env.out) == "yes", "an environment given replaces this process's");
+    }
+
+    {
+        // An fd this process left open without O_CLOEXEC must not reach the child.
+        const std::int32_t leaked = open("/dev/null", O_RDONLY);
+        const std::int32_t high = dup2(leaked, 57);
+        const Process::Captured fds = Process::Run({ "sh", "-c", "ls /proc/$$/fd" });
+        const std::vector<std::string> open = Dyn::Split(fds.out);
+        Check(std::ranges::find(open, "57") == open.end() && open.size() <= 4, std::format("only the three streams reach the child (it had: {})", fds.out));
+        close(high);
+        close(leaked);
+    }
+
+    {
+        const Process::Captured status = Process::Run({ "sh", "-c", "cat /proc/self/status" });
+        const std::uint64_t ignored = HexMask(status.out, "SigIgn:");
+        const std::uint64_t blocked = HexMask(status.out, "SigBlk:");
+        Check((ignored & (1ULL << (SIGPIPE - 1))) == 0, "the child has SIGPIPE back at its default");
+        Check(blocked == 0, "the child's signal mask is empty");
+    }
+
+    {
+        // The target exits and leaves a detached grandchild behind.
+        const fs::path pidFile = dir / "left-behind";
+        const Process::Child supervisor = StartSupervised(std::format("setsid sleep 300 & echo $! > {}", pidFile.string()));
+        const std::int64_t grandchild = PidFrom(pidFile);
+        const std::optional<std::int32_t> code = Process::Wait(supervisor.pid, 10s);
+        Check(grandchild > 0 && code == 0, "a supervised target's exit status comes back");
+        Check(GoneWithin(grandchild, 2s), "what a target leaves behind dies with it, a detached process group included");
+    }
+
+    {
+        // A timeout or an abort: SIGTERM to the supervisor.
+        const fs::path pidFile = dir / "terminated";
+        const Process::Child supervisor = StartSupervised(std::format("setsid sleep 300 & echo $! > {}; sleep 300", pidFile.string()));
+        const std::int64_t grandchild = PidFrom(pidFile);
+        Check(grandchild > 0 && Alive(grandchild), "the grandchild runs while the target does");
+        Process::Signal(supervisor.pid, SIGTERM);
+        const std::optional<std::int32_t> code = Process::Wait(supervisor.pid, 10s);
+        Check(code.has_value() && Process::SupervisedReturnCode(*code) == -SIGKILL, std::format("a stopped target reports the kill (supervisor exit {})", code.value_or(-1)));
+        Check(GoneWithin(grandchild, 2s), "SIGTERM to the supervisor takes the whole tree down");
+    }
+
+    {
+        // The driver itself is SIGKILLed: nothing of the pass may outlive it.
+        const fs::path pidFile = dir / "driver-killed";
+        const Process::Child driver = Process::Start({ .argv = { Platform::SelfExe().string(), "--as-driver", pidFile.string() } });
+        const std::int64_t grandchild = PidFrom(pidFile);
+        Check(grandchild > 0 && Alive(grandchild), "the driver's pass is running");
+        Process::Signal(driver.pid, SIGKILL);
+        Process::Wait(driver.pid, 10s);
+        Check(GoneWithin(grandchild, 3s), "a SIGKILLed driver's pass dies with it, through the supervisor's death signal");
+    }
+
+    {
+        const Process::Child running = Process::Start({ .argv = { "sleep", "5" } });
+        Check(!Process::Wait(running.pid, 100ms).has_value(), "a wait that times out leaves the child running");
+        Process::Signal(running.pid, SIGKILL);
+        Check(Process::Wait(running.pid, 5s) == -SIGKILL, "and the child can still be waited for");
+        Check(Process::SupervisedReturnCode(137) == -9 && Process::SupervisedReturnCode(3) == 3, "a supervisor's 128+n is signal n");
+    }
+
+    fs::remove_all(dir);
+    return Failures == 0 ? 0 : 1;
+}