whisper : add stateless VAD detect + explicit state reset for streaming (#3677)

whisper : add stateless VAD detect + explicit state reset for streaming (#3677)

whisper_vad_detect_speech resets LSTM state on every call, which is
correct for batch processing but prevents temporal continuity when
calling per-chunk in a streaming loop.

Add whisper_vad_detect_speech_no_reset (skips buffer clear) and
whisper_vad_reset_state (explicit clear between utterances).
Existing whisper_vad_detect_speech is now a thin wrapper — zero
behavior change for current callers.

Co-authored-by: Claude Opus 4.6 (1M context) <redacted>