fix: retire AVAudioEngine after configuration changes

[jatinkrmalik]

Summary

Fixes the rest of #196. The HFP settle wait in #197 got Soundcore recordings to start, but later reports from @cbauer42 show the same path still crashing and interrupting recordings.

Crash

handleAudioConfigurationChange called releaseEngine(), which dropped the last reference to AVAudioEngine while AVFAudio was still in AVAudioIOUnit::IOUnitPropertyListener. That is the EXC_BAD_ACCESS on macOS 26.6 and 26.6.1. Apple documents this notification as: do not deallocate the engine from the client's handler.

Erratic recordings

Bluetooth A2DP/HFP switches keep posting configuration changes after start. We treated those as a lost microphone, force-stopped, and destroyed the engine. That matches the menu bar audio icon appearing and disappearing, Music.app launching on its own, and CADefaultDeviceAggregate showing up in the dropdown.

Fix

• Detach the invalidated engine immediately so the next recording cannot reuse it, but keep the old AVAudioEngine alive for idleEngineReleaseDelay so AVFAudio can finish its callbacks.
• If we are still recording and the graph actually stopped, rebuild the tap on the existing engine instead of aborting the session.
• Hide CADefaultDeviceAggregate from the mic picker and treat it as a healthy transient route while HFP is settling.

Tests

Unit coverage for aggregate detection, picker filtering, route health, and the startup ignore window.

Hardware check

Please try a Bluetooth headset (Soundcore Liberty 5 / AirPods):

1. Push-to-talk should record without a red flash or a crash.
2. Repeat PTT a few times. The menu bar audio icon should stay put instead of flickering.
3. CADefaultDeviceAggregate should not appear in the mic dropdown.
4. Unplugging a wired mic mid-recording should still stop and recover.

  

[cbauer42]

Tried the nightly but now it doesn't work at all. So my guess is that you have a fundamental issue here that prevents the app from working when audio devices change on macOS. I plug in my audio interface or connect my BT headset -> weird things happen.

[jatinkrmalik]

@cbauer42 Did you try this build because I just built it with this commit? https://github.com/VocaHQ/vocamac/releases/tag/nightly ?