feat(script): scripts in project.cpp, run with crafter-build run

A project.cpp could describe how a project is built, tested and linted,
but not the chores around it: packaging, installing, publishing. Those
stayed shell lines in a README or in someone's history.

Configuration gains `scripts`, a plain list like `defines`. A Script is
a name and a C++ callback, as a lint rule is; there is no script
language and no config file. The callback gets a ScriptContext (the
project, the project file's directory, the arguments) and returns the
exit status. ctx.Run runs a shell command with the terminal attached
and returns its status. It is only stored when project.cpp loads, which
happens for every verb and for every project that depends on this one.

`crafter-build run <name> [args...]` runs one, a bare `run` lists them.
Only the scripts of the project crafter-build was pointed at are
runnable, never a dependency's.

Everything after the script's name is handed to the script untouched.
The two branches that do this sit ahead of every other branch of the
parse loop, --version and --help included. Below the verbs,
`run package clean` would delete build/, and any other word would land
in the project's args and so in the variant hash, moving the build to
another directory. Options before the name keep their meaning, so
project args go there.

The driver is its own partition, :Script. It refuses a project with no
scripts, a name that is empty or starts with '-' (it could never be
typed), a name declared twice (running either would be a guess) and an
unknown name, each with exit 1 and without running anything. A script
that throws exits 1. A return value outside 0..255 becomes 1: an exit
status is eight bits, and a raw wait status such as 256 would otherwise
wrap to success.

Two helpers in :Platform:

- RunCommandAttached runs a command with inherited stdio. Every helper
  so far captured through popen, which is right for a compile step and
  useless for a command that runs for half an hour or asks a question.
  It reads the wait status through DecodeWaitStatus, which
  RunCommandChecked and RunCommandWithTimeout now share.
- ShellQuote is exported. It existed twice, as ShellQuote in :External
  and ShellQuoteSh in :Test, with the same behaviour; both are gone. A
  script that puts a path into a command needs it too.

Not included: a string shorthand for one-command scripts (a one-line
lambda covers it) and a ctx.Build() helper (Build() is already
exported).

A new partition and new exported symbols: run ./build.sh.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

implementations/Crafter.Build-Platform.cpp

@@ -552,6 +552,35 @@ bool Crafter::IsTopLevelProject() {
     return LoadingFrames.size() <= 1;
 }
 
+namespace {
+    // The one reading of a wait status (pclose, system) for every helper
+    // that runs a command: a signal death is reported the way a shell
+    // reports it, as 128+signal.
+    void DecodeWaitStatus(std::int32_t status, CommandResult& result) {
+        if (WIFEXITED(status)) {
+            result.exitCode = WEXITSTATUS(status);
+        } else if (WIFSIGNALED(status)) {
+            result.signal = WTERMSIG(status);
+            result.exitCode = 128 + result.signal;
+            result.crashed = true;
+        } else {
+            result.exitCode = -1;
+        }
+    }
+}
+
+std::string Crafter::ShellQuote(std::string_view s) {
+    std::string out;
+    out.reserve(s.size() + 2);
+    out.push_back('\'');
+    for (char c : s) {
+        if (c == '\'') out += "'\\''";
+        else out.push_back(c);
+    }
+    out.push_back('\'');
+    return out;
+}
+
 CommandResult Crafter::RunCommandChecked(std::string_view cmd) {
     std::array<char, 128> buffer;
     CommandResult result{};
@@ -564,19 +593,27 @@ CommandResult Crafter::RunCommandChecked(std::string_view cmd) {
         result.output += buffer.data();
     }
 
-    std::int32_t status = pclose(pipe);
-    if (WIFEXITED(status)) {
-        result.exitCode = WEXITSTATUS(status);
-    } else if (WIFSIGNALED(status)) {
-        result.signal = WTERMSIG(status);
-        result.exitCode = 128 + result.signal;
-        result.crashed = true;
-    } else {
-        result.exitCode = -1;
-    }
+    DecodeWaitStatus(pclose(pipe), result);
     return result;
 }
 
+std::int32_t Crafter::RunCommandAttached(std::string_view cmd) {
+    // The child writes straight to the terminal. Anything of ours that is
+    // still pending — the in-place status line, buffered stdout when it is
+    // a pipe — has to be out before the child's first byte, or it lands in
+    // the middle of the child's output or after it.
+    Progress::Clear();
+    std::fflush(stdout);
+    std::fflush(stderr);
+
+    const std::string command(cmd);
+    const std::int32_t status = std::system(command.c_str());
+    if (status == -1) throw std::runtime_error(std::format("could not start a shell for: {}", cmd));
+    CommandResult result{};
+    DecodeWaitStatus(status, result);
+    return result.exitCode;
+}
+
 CommandResult Crafter::RunCommandWithTimeout(std::string_view cmd, std::chrono::seconds timeout) {
     std::array<char, 128> buffer;
     CommandResult result{};
@@ -592,25 +629,16 @@ CommandResult Crafter::RunCommandWithTimeout(std::string_view cmd, std::chrono::
         result.output += buffer.data();
     }
 
-    std::int32_t status = pclose(pipe);
-    if (WIFEXITED(status)) {
-        std::int32_t code = WEXITSTATUS(status);
-        if (code == 124) {
+    DecodeWaitStatus(pclose(pipe), result);
+    // What comes back is `timeout`'s own exit, and it speaks through the
+    // code: 124 for a command it had to stop, 128+signal for one that died.
+    if (!result.crashed) {
+        if (result.exitCode == 124) {
             result.timedOut = true;
-            result.exitCode = 124;
-        } else if (code >= 128) {
-            result.signal = code - 128;
-            result.exitCode = code;
+        } else if (result.exitCode >= 128) {
+            result.signal = result.exitCode - 128;
             result.crashed = true;
-        } else {
-            result.exitCode = code;
         }
-    } else if (WIFSIGNALED(status)) {
-        result.signal = WTERMSIG(status);
-        result.exitCode = 128 + result.signal;
-        result.crashed = true;
-    } else {
-        result.exitCode = -1;
     }
     return result;
 }
@@ -736,7 +764,7 @@ std::string Crafter::GetBaseCommand(const Configuration& config) {
 }
 
 namespace {
-    constexpr std::array<std::string_view, 12> CrafterBuildModules = {
+    constexpr std::array<std::string_view, 13> CrafterBuildModules = {
         "Crafter.Build-Toolchain",
         "Crafter.Build-Shader",
         "Crafter.Build-Platform",
@@ -746,6 +774,7 @@ namespace {
         "Crafter.Build-Clang",
         "Crafter.Build-Test",
         "Crafter.Build-Lint",
+        "Crafter.Build-Script",
         "Crafter.Build-Progress",
         "Crafter.Build-Asset",
         "Crafter.Build",