- Add model file existence check and ONNX error output in gunshot_classifier
- Add 3 new unit tests to test_core_lib:
- audio_buffer_overflow: verifies capacity capping behavior
- distance_estimator_multi_class: verifies gunshot/artillery/explosion refs
- threat_tracker_filtering: verifies min_event_interval deduplication
- Add .gitattributes to mark .onnx, .bin, .wav as binary files
- All 9 core tests pass
