Add real-time speaker diarization support

- New whisper_live/diarization.py: SpeakerDiarizer with online clustering
- Uses pyannote.audio speaker embeddings (optional dependency)
- Cosine similarity threshold for speaker matching (default 0.55)
- Running average embedding update for speaker stability
- Configurable max_speakers limit (default 10)
- Client options: enable_diarization, max_speakers
- Segments include 'speaker' field when diarization is active
- Graceful fallback: logs warning if pyannote not installed
- Added 12 unit tests (mock-based, no GPU required)

tests/test_client.py

@@ -56,6 +56,8 @@ class TestClientCallbacks(BaseTestCase):
             "enable_translation": False,
             "target_language": "fr",
             "hotwords": None,
+            "enable_diarization": False,
+            "max_speakers": 10,
         })
         self.client.on_open(self.mock_ws_app)
         self.mock_ws_app.send.assert_called_with(expected_message)