* fix(audio): prevent Bluetooth mic probe heap corruption
Open PortAudio streams only once when negotiating capture format, and
always stop_stream() before close(). Separate channel/rate probing was
aborting on Bluetooth SCO devices (e.g. FreeBuds) with malloc corruption.
Fixes#567
* refactor(audio): open capture stream once instead of probe-close-reopen
Adversarial review of the first fix found it still closed the probe
stream and immediately reopened the device — the same rapid open/close
pattern blamed for the Bluetooth SCO heap corruption, just fewer cycles.
_open_capture_stream() now returns the successfully negotiated stream
and all three callers (mic test, recording, reconnect) use it directly,
so each capture session performs exactly one PortAudio open. Also drops
the dead known_working_rate parameter.
* test(audio): cover negotiation fallback paths for codecov patch
Add tests for mic-test, recording, and reconnect fallback branches when
_open_capture_stream returns no stream, plus Bluetooth settle-on-failure
and empty-read reconnect cleanup. Brings patch diff coverage above 80%.
