fix: rebuild and relink on what changed, submodules at their revisions, and mingw found where it is

- Shaders record the files glslang opened (a depfile) and their settings
  (source path, stage, entry point, defines, include directories) in the
  build directory, and rebuild when an included file or a setting changes,
  not only the shader's own source.
- An executable relinks when its link command changed, not only when an
  object is newer: a link flag in project.cpp, or a dependency's library
  that moved (an ExternalDependency built into a new cache directory). The
  command is recorded beside the objects; its flags come from ordered sets,
  so the order the dependency threads finish in can't change it.
- An external's newest artifact counts its libDirs and versioned .so names.
- Submodules are checked out at the revisions the fetched commit records,
  from the URLs its .gitmodules gives, after every clone, switch, checkout
  and pull, so a pinned commit builds against its own submodules, and one
  moved by hand is put back; with no .gitmodules, git isn't asked.
- Each external clone's fetch and build hold a file lock: parallel builds
  pulled into one clone at once ("cannot lock ref").
- mingw:
  - -stdlib=libstdc++ always;
  - the std module's source is found through libstdc++.modules.json in the
    mingw toolchain's own library directories (the generic lib/ holds the
    host's Linux one), asking the compiler once a run;
  - a toolchain that isn't there is a build error saying where it looked,
    not std::terminate;
  - CRAFTER_MINGW_DIR names the prefix, the directory holding
    x86_64-w64-mingw32/ and lib/gcc/;
  - inside the Flatpak the host's /usr mingw is used.
- RunScript's exit-status check reads as the one range it is.
- Tests: ConcurrentExternalFetch, ExternalArtifactTime, GitSubmodules,
  LinkFlagsChange, MingwMissing, ShaderIncludeChange; ShaderCompile and
  ToolchainResolve extended.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/ConcurrentExternalFetch/main.cpp

@@ -0,0 +1,95 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// Builds that run at the same time share their dependencies' clones in the
+// cache: the tests `crafter-build test --jobs=N` builds side by side each
+// declare the same ExternalDependency, and two crafter-builds can run at
+// once. Fetching a clone that already exists ran `git pull` (and the CMake
+// build) in it with nothing to keep a second fetch out, and the second git
+// fails on the first one's index.lock. A clone's fetch and build now hold a
+// lock of their own.
+//
+// Eight threads fetch the same tracked branch of a local repository at
+// once, a few rounds over, the branch moving between rounds so every pull
+// has work to do; every fetch has to succeed. Everything is local;
+// XDG_CACHE_HOME keeps the clone out of the real cache.
+
+extern "C" int setenv(const char* name, const char* value, int overwrite);
+
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+
+    void Git(const fs::path& dir, std::string_view args) {
+        const std::string command = std::format("git -C {} {}", ShellQuote(dir.string()), args);
+        auto result = RunCommandWithTimeout(command, std::chrono::seconds(60));
+        if (result.exitCode != 0) throw std::runtime_error(std::format("{} failed: {}", command, result.output));
+    }
+
+    void Commit(const fs::path& repo, std::uint32_t round) {
+        std::ofstream(repo / "round.txt", std::ios::binary | std::ios::trunc) << round;
+        Git(repo, "add round.txt");
+        Git(repo, std::format("commit -q -m round-{}", round));
+    }
+}
+
+int main() {
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-concurrent-external-fetch";
+    fs::remove_all(scratch);
+    fs::create_directories(scratch);
+    setenv("XDG_CACHE_HOME", (scratch / "cache").c_str(), 1);
+    setenv("GIT_CONFIG_NOSYSTEM", "1", 1);
+    setenv("GIT_CONFIG_GLOBAL", "/dev/null", 1);
+    for (const char* name : { "GIT_AUTHOR_NAME", "GIT_COMMITTER_NAME" }) setenv(name, "crafter-build test", 1);
+    for (const char* name : { "GIT_AUTHOR_EMAIL", "GIT_COMMITTER_EMAIL" }) setenv(name, "test@example.invalid", 1);
+
+    const fs::path repo = scratch / "shared";
+    fs::create_directories(repo);
+    Git(repo, "init -q -b main");
+    Commit(repo, 0);
+    const GitSource source { .url = std::format("file://{}", repo.string()), .branch = "main" };
+    const fs::path clone = GitFetch(source);
+
+    constexpr std::uint32_t threads = 8;
+    constexpr std::uint32_t rounds = 4;
+    std::vector<std::string> errors;
+    std::mutex errorsMutex;
+    for (std::uint32_t round = 1; round <= rounds; ++round) {
+        Commit(repo, round);
+        std::vector<std::jthread> fetchers;
+        for (std::uint32_t i = 0; i < threads; ++i) {
+            fetchers.emplace_back([&] {
+                try {
+                    (void)GitFetch(source);
+                } catch (const std::exception& e) {
+                    std::lock_guard lock(errorsMutex);
+                    errors.push_back(e.what());
+                }
+            });
+        }
+    }
+    Check(errors.empty(), std::format("{} fetches at once, {} rounds: every one succeeds ({} failed{}{})", threads, rounds, errors.size(), errors.empty() ? "" : ", the first: ", errors.empty() ? "" : errors.front()));
+    std::ifstream round(clone / "round.txt");
+    std::uint32_t last = 0;
+    round >> last;
+    Check(last == rounds, std::format("and the clone ends at the last round ({})", last));
+
+    std::error_code ec;
+    fs::remove_all(scratch, ec);
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}