TantivyBackend now uses open()/close()/_ensure_open() instead of __enter__/__exit__.
get_backend() tracks _backend_path and auto-reinitializes when settings.INDEX_DIR
changes, fixing the xdist/override_settings isolation bug where parallel workers
would share a stale singleton pointing at a deleted index directory.
Test fixtures use in-memory indices (path=None) for speed and isolation.
Singleton behavior covered by TestSingleton in test_backend.py.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>