Detect click/pop discontinuities with bounded PCM evidence

[9o6]

Closes #4

Summary

• add a deterministic PCM16 adjacent-sample discontinuity detector across chunk boundaries
• preserve stage media time separately from wall-clock event time and deduplicate adjacent detections
• project bounded incidents with optional nearby queue-clear/barge-in evidence and an explicit remote playout boundary
• document the detector contract, thresholds, silence handling, and proof limitations

Testing

• python3 -m ruff check src/voxbench/observability/observer.py src/voxbench/observability/__init__.py src/voxbench/control_plane/run_api.py tests/test_observability.py
• python3 -m pytest -q (330 passed, 4 skipped)

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[engkimo]

The detector contract and bounded evidence projection look good, and the full test/build suite passes. One concurrency issue should be fixed before merge.

observe_stage_audio() reads _stage_pcm_state before acquiring _lock, then later updates that state inside the lock. Concurrent calls for the same stage can both consume the same previous sample/frame offset and overwrite each other, which can lose a chunk-boundary discontinuity, roll back media time, or bypass the 5 ms dedup state. Since this is stateful cross-chunk analysis, please serialize the read → detect → state update sequence for each stage (or otherwise make it atomic), and add a focused concurrent-call regression test.

[9o6]

Addressed the concurrency issue in 394c76a. The per-stage PCM state read, discontinuity detection, dedup decision, media-frame advancement, and state update now execute in one observer lock section.

I also added a focused concurrent-call regression test that pauses the first detector invocation, starts a second call for the same stage, verifies the detector invocations are serialized, and confirms both boundaries retain monotonic media times (5 ms and 10 ms).

Validation:

• ruff check .
• pytest -q — 331 passed, 4 skipped

[engkimo]

Re-reviewed the concurrency fix. The stage PCM state read, discontinuity detection, dedup decision, and state update are now serialized under the observer lock, and the focused concurrent-call regression test verifies that the second call cannot consume stale state. The observability tests pass locally (29 passed), and the required Python, web build, and security checks are green. Approved.