Fix: microphone capture dies on input format/route change (Meet, earbuds)

[morellid]

Problem

MicCapture reads the input format once at start, creates the AVAudioFile in that format, and writes raw tap buffers directly. If the input device is reconfigured mid-stream, the tap buffers stop matching the file's format and every write(from:) fails with CoreAudio -50 (paramErr), so the microphone track dies silently for the rest of the recording.

Two common triggers:

1. A conferencing app (Google Meet, Zoom, Teams) opens the mic for WebRTC and forces the shared input device into voice-processing mode (e.g. 3-channel / 48 kHz). Core Audio shares the device's nominal format across clients, so the already-open engine tap starts receiving a format the file was not created for.
2. The default input device changes during recording (plugging in earbuds / a headset).

In both cases --mic capture produced only the audio recorded before the change.

Reproduction

1. ownscribe ... --mic (start recording).
2. Speak.
3. Join a Google Meet call.
4. Speak again.
5. Stop. Only the pre-call speech is in the merged output; stderr shows repeated -50 write errors.

Fix

Two independent safety nets, both mirroring patterns already used by SystemAudioCapture in the same file:

• Format normalisation. writeBuffer(_:) converts every tap buffer to the file's fixed processingFormat through an AVAudioConverter (rebuilt when the incoming format changes), using the block-based convert(to:error:withInputFrom:) form so sample-rate / channel-count changes are handled. A fast path writes directly when the format is unchanged, so the steady-state path is unchanged from before.
• Route-change handling. An AVAudioEngineConfigurationChange observer (delivered on .main, so it serialises with stop()) removes the tap, re-applies a named device if one was requested, reinstalls the tap, and restarts the engine. A transient zero sample-rate while the route settles is retried on .main (bounded) rather than relying on a second notification, so one transient change cannot leave the mic dead.

Also removed an inputNode.audioUnit! force-unwrap that could crash during route churn.

Testing

• Builds clean with swift/build.sh (Swift 6.2, macOS 26).
• Verified end-to-end: recorded with --mic, joined a Google Meet call mid-recording, and confirmed both the pre-call and post-call speech are present in the merged output (previously only pre-call survived).

Notes / deliberate tradeoffs

• The mic file is written continuously with no silence padding across a route change, so a brief input dropout during a device switch (earbuds) leaves the mic track slightly time-shifted relative to the system track. This is acceptable for transcription/diarisation and is far better than losing the audio. The conferencing-app case reconfigures only the format (not the device), so the converter absorbs it with no gap.
• A sub-buffer of converter tail is dropped when the converter is rebuilt on a format change (sub-10 ms, inaudible).

🤖 Generated with Claude Code