- register() now holds _plugins_lock when writing to _REGISTRY, preventing
concurrent write races if plugins are registered outside the bootstrap path
- _ensure_plugins_loaded() now sets _plugins_load_failed=True on ImportError
so subsequent lookups return None immediately instead of retrying the import
on every call
- _isolated_registry fixture in test_registry.py resets _plugins_load_failed
- Two new tests cover the circuit-breaker skip path and the failure-flag path
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>