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>

interfaces/Crafter.Build-Platform.cppm

@@ -14,6 +14,21 @@ namespace Crafter {
         bool timedOut = false;
         std::int32_t signal = 0;
     };
+    // An exclusive lock on the file `path` (created if missing), held from
+    // construction to destruction. An flock, so it keeps out the other
+    // threads of this process (each opens the file anew) and other
+    // crafter-build processes alike. The OS drops it when the descriptor
+    // closes, process death included, so an interrupted build never leaves a
+    // stale lock; the descriptor is close-on-exec, so the tools a holder
+    // spawns don't keep it past its release.
+    class FileLock {
+        std::int32_t fd = -1;
+    public:
+        explicit FileLock(const fs::path& path);
+        ~FileLock();
+        FileLock(const FileLock&) = delete;
+        FileLock& operator=(const FileLock&) = delete;
+    };
     std::string BuildStdPcm(const Configuration& config, fs::path stdPcm);
     fs::path GetCacheDir();
     CommandResult RunCommandChecked(std::string_view command);