fix: restore hot-mic with correct prerequisites
Now that we know the EIO root causes (HDMI audio + 6-period playback
buffer), hot-mic can work safely with:
- No HDMI PCM open in audio_init
- 3-period playback buffer (original value)
- Capture mixer enabled in capture thread (not audio_init)
- ALSA default period/buffer (no explicit setting)
This gives instant recording with zero device-open latency.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>