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/voice_to_notes/services/transcribe.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 import sys
 import time
+from collections.abc import Callable
 from dataclasses import dataclass, field
 from typing import Any
 
@@ -90,6 +91,7 @@ class TranscribeService:
         device: str = "cpu",
         compute_type: str = "int8",
         language: str | None = None,
+        on_segment: Callable[[SegmentResult, int], None] | None = None,
     ) -> TranscriptionResult:
         """Transcribe an audio file with word-level timestamps.
 
@@ -145,6 +147,9 @@ class TranscribeService:
                 )
             )
 
+            if on_segment:
+                on_segment(result.segments[-1], segment_count - 1)
+
             # Send progress every few segments
             if segment_count % 5 == 0:
                 write_message(