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>

implementations/Crafter.Build-Toolchain.cpp

@@ -137,9 +137,10 @@ Toolchain Crafter::ResolveToolchain(const ToolchainInputs& in) {
     }
     if (std::string_view dir = Env(in, "CRAFTER_MINGW_DIR"); !dir.empty()) {
         tc.mingwSysroot = fs::path(dir);
-        tc.mingwSysrootExplicit = true;
-    } else {
-        tc.mingwSysroot = "/usr/x86_64-w64-mingw32";
+    } else if (!in.hostRoot.empty() && fs::exists(in.hostRoot / "usr" / "x86_64-w64-mingw32")) {
+        // The sandbox has no mingw-w64 of its own, and no mingw gcc on PATH
+        // for clang to find one by.
+        tc.mingwSysroot = in.hostRoot / "usr";
     }
     return tc;
 }
@@ -157,6 +158,7 @@ const Toolchain& Crafter::GetToolchain() {
         try {
             in.crafterPrefix = GetCrafterBuildHome().parent_path().parent_path();
         } catch (const std::exception&) {}
+        if (fs::exists("/.flatpak-info")) in.hostRoot = "/run/host";
 
         Toolchain tc = ResolveToolchain(in);
         if (!tc.error.empty()) return tc;