All 5 functions now have proper #region/#endregion contracts:
- _detector_cache_key [C:1] — private helper
- build_detector [C:2] — public API
- get_detector [C:2] — public API with caching
- detect_language [C:2] — public API, @RAISES for safety
- batch_detect [C:3] — public API with @RELATION edges
Clean audit: 0 warnings on _lang_detect.py