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/ExternalArtifactTime/main.cpp

@@ -0,0 +1,89 @@
+// 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;
+
+// A CMake dependency's newest library decides when its consumers are stale
+// (BuildExternal's latestArtifact). Only files directly in the CMake build
+// directory named *.a or *.so were looked at, so msquic's
+// bin/Release/libmsquic.so.2, in one of the dependency's libDirs and with a
+// version after the .so, never counted: a rebuilt msquic left everything
+// built against the old one in place.
+//
+// The dependency here is a local repository whose CMake project puts a
+// versioned shared library in bin/Release, declared as a libDir. 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 Write(const fs::path& file, std::string_view text) {
+        std::ofstream(file, std::ios::binary | std::ios::trunc) << text;
+    }
+}
+
+int main() {
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-external-artifact-time";
+    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 / "versioned";
+    fs::create_directories(repo);
+    Write(repo / "lib.c", "int Versioned(void) { return 2; }\n");
+    Write(repo / "CMakeLists.txt",
+          "cmake_minimum_required(VERSION 3.16)\n"
+          "project(versioned C)\n"
+          "add_library(versioned SHARED lib.c)\n"
+          "set_target_properties(versioned PROPERTIES VERSION 2.0.0 SOVERSION 2\n"
+          "    LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin/Release)\n");
+    Git(repo, "init -q -b main");
+    Git(repo, "add .");
+    Git(repo, "commit -q -m versioned");
+
+    ExternalDependency dep;
+    dep.name = "versioned";
+    dep.source.url = std::format("file://{}", repo.string());
+    dep.builder = ExternalBuilder::CMake;
+    dep.libDirs = { "bin/Release" };
+    dep.libs = { "versioned" };
+    std::atomic<bool> cancelled = false;
+    const ExternalBuildResult result = BuildExternal(dep, HostTarget(), cancelled);
+    Check(result.error.empty(), std::format("the dependency builds: {}", result.error));
+
+    const fs::path library = ExternalCloneDir(dep, HostTarget()) / "build" / "bin" / "Release" / "libversioned.so.2.0.0";
+    Check(fs::exists(library), std::format("the library is where its CMake project puts it: {}", library.string()));
+    if (fs::exists(library)) {
+        Check(result.latestArtifact == fs::last_write_time(library), "its newest artifact is the versioned library in the libDir");
+    }
+
+    std::error_code ec;
+    fs::remove_all(scratch, ec);
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}