test: a std module rebuilt in place recompiles the units importing it

StdPcmRebuild builds a project whose main.cpp imports std, then dates
the source before the object and the object before std.pcm, as a std
module rebuilt after the object leaves them, so that nothing else makes
the object stale. The next build has to compile main.cpp again, and the
idle build before it must compile nothing. Without -fmodule-file-deps
the object was only relinked. The test redirects the cache, since
redating a std PCM the real cache shares would rebuild its other users.

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

tests/StdPcmRebuild/fixture/main.cpp

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+
+int main() {
+    std::println("rebuilt");
+}