fix: atomic-rename host-cache PCMs to close concurrent-build race

Two crafter-build invocations sharing XDG_CACHE_HOME used to clobber each
other's writes to <cache>/<target>-<march>/std.pcm and the
Crafter.Build-*.pcm modules: each LoadProject path wrote directly to the
final path, so a reader could see a half-written file and die with
"malformed or corrupted precompiled file: 'can't skip to bit X from Y'"
(issue #14). Every BuildStdPcm / EnsureCrafterBuildPcms write now goes via
<final>.tmp.<pid>.<seq> and atomic-renames into place; concurrent writers
always see either the old or the new file, never torn bytes. The mingw-on-
Linux std.cppm copy is per-PID for the same reason. Adds a regression test
(ConcurrentCacheRace) that races four LoadProject() calls against a cold
scratch cache — reproduces the race 5/5 without the fix and passes 5/5
with it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

project.cpp

@@ -107,6 +107,12 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
         cfg.AddTest("VariantId").Dependencies({ crafterBuildLib.get() });
         cfg.AddTest("WasiBrowserRuntime").Dependencies({ crafterBuildLib.get() });
         cfg.AddTest("RunSingleTestExit").Dependencies({ crafterBuildLib.get() });
+        // LoadProject dlopens the synthesized project.so, which references
+        // Crafter:: symbols (HostTarget, Configuration ctors) that have to be
+        // visible from the test exe — same wiring crafter-build itself uses
+        // for project.so.
+        cfg.AddTest("ConcurrentCacheRace").Dependencies({ crafterBuildLib.get() })
+            .LinkFlag("-Wl,--export-dynamic").LinkFlag("-ldl");
     }
 
     return cfg;