TDD: 3 tests covering silence passthrough, speech→silence segment emission,
and short-speech discard. Uses _speech_ms tracking (not total buffer length)
for accurate min_speech_ms enforcement. Silero VAD import is try/except'd
so tests run without torch via mocker.patch on vad_prob.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
