fix: recover audio engine after route changes

[jatinkrmalik]

Summary

Fix recovery from macOS audio route/input format churn by making the recording start path report failure and rebuilding the AVAudioEngine when the current instance may hold stale hardware format state.

This is aimed at failures like:

AVAudioEngine exception while starting recording: Input HW format and tap format not matching

which can happen after Bluetooth headset connect/disconnect, default input changes, Zoom/Loom microphone activity, or other AVAudioEngine configuration changes.

Changes

• Return a success/failure result from AudioRecording.startRecording(...).
• Reset AppState and hotkey state immediately if AudioEngine fails to start.
• Recreate the underlying AVAudioEngine after:
  • audio configuration changes
  • failed start attempts
  • force reset
• Keep AVAudioEngineConfigurationChange scoped to the current engine and re-register the observer when the engine is recreated.
• Add a regression test for failed audio start resetting recording and hotkey state.

Validation

• swift test --filter AppStateRecordingGuardTests/testFailedAudioStartResetsRecordingAndHotkeyState
• swift build
• swift test — 169 tests passed

Test notes

Please try this build with Bluetooth headphones connect/disconnect and during/after Zoom or Loom microphone usage to verify VocaMac no longer needs an app restart after input route changes.

[jatinkrmalik]

Closed as superseded.

Current main already has the safer lazy audio-engine lifecycle, start-failure reset, and fixed input-device support. This PR now conflicts and would regress that model.