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>
