Keep unfinished speech turns fail closed

[RobVanProd]

Outcome

Keeps interrupted, stalled, expired, and incomplete microphone turns fail closed. Only a valid VAD trailing-silence terminal can reach Whisper; cancellation clears host PCM and runs zero STT/model/TTS.

Firmware

• Measures endpoint timing from captured PCM, not missing wall time.
• Uses nonblocking socket writes with retained terminal retry/cancel semantics.
• Cancels on mic stalls, chunk discontinuity, backpressure, expiry, or terminal failure.
• Adds phase, terminal, and write-deferral telemetry.

Host

• Adds a 14.5 s absolute capture lease and 4 s inactivity lease.
• Handles utterance_cancel idempotently and rejects stale/mismatched ends.
• Clears partial PCM on cancellation, expiry, supersession, and disconnect.

Verification

• pio test -e native_logic: 313/313 pass
• python -m unittest discover -s bridge -p test_*.py: 584/584 pass
• python -m py_compile bridge/lan_service.py: pass
• pio run -e stackchan_release_full: pass
• private stackchan_release_forensics build: pass, SHA-256 f1a6319fb1d6703c48f2d248c962ce4a267a7eed1db83b894e587b0933f370c2

Physical deployment and completed/incomplete utterance evidence remain pending and will be added without treating simulated evidence as physical.