pytest_plugin(feat): Add XDG persistent cache and fixture optimization

why: Improve test performance with session-scoped caching and atomic
initialization for pytest-xdist compatibility.

what:
- Add XDG-based persistent cache for fixture master copies
- Add RepoFixtureResult dataclass with cache hit/miss tracking
- Add atomic_init for race-free initialization in parallel tests
- Optimize git_repo, hg_repo, svn_repo fixtures with caching

Author: tony