Stream transcript segments to frontend as they are transcribed

Send each segment to the frontend immediately after transcription via
a new pipeline.segment IPC message, then send speaker assignments as a
batch pipeline.speaker_update message after diarization completes. This
lets the UI display segments progressively instead of waiting for the
entire pipeline to finish.

Changes:
- Add partial_segment_message and speaker_update_message IPC factories
- Add on_segment callback parameter to TranscribeService.transcribe()
- Emit partial segments and speaker updates from PipelineService.run()
- Add send_and_receive_with_progress to SidecarManager (Rust)
- Route pipeline.segment/speaker_update events in run_pipeline command
- Listen for streaming events in Svelte frontend (+page.svelte)
- Add tests for new message types, callback signature, and update logic

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

python/tests/test_pipeline.py

@@ -88,3 +88,18 @@ def test_merge_results_no_speaker_segments():
 
     result = service._merge_results(transcription, [])
     assert result.segments[0].speaker is None
+
+
+def test_speaker_update_generation():
+    """Test that speaker updates are generated after merge."""
+    result = PipelineResult(
+        segments=[
+            PipelineSegment(text="Hello", start_ms=0, end_ms=1000, speaker="SPEAKER_00"),
+            PipelineSegment(text="World", start_ms=1000, end_ms=2000, speaker="SPEAKER_01"),
+            PipelineSegment(text="Foo", start_ms=2000, end_ms=3000, speaker=None),
+        ],
+    )
+    updates = [{"index": i, "speaker": seg.speaker} for i, seg in enumerate(result.segments) if seg.speaker]
+    assert len(updates) == 2
+    assert updates[0] == {"index": 0, "speaker": "SPEAKER_00"}
+    assert updates[1] == {"index": 1, "speaker": "SPEAKER_01"}