Fix cannotSetupAudioInputs when external USB audio device is connected before app launch

[mpogra]

Problem

When an external USB audio device (e.g. a microphone via Lightning or USB-C adapter) is connected before the app launches, AVCaptureSession initialises its audio subsystem in a degraded state. The CMSampleBuffer format description produced in this state has:

• No AudioChannelLayout (CMAudioFormatDescriptionGetChannelLayout returns nil)
• Potentially zero values for sample rate and channel count in the ASBD

The original code always set:

audioSettings[AVChannelLayoutKey] = Data()   // empty Data when layout is absent

AVAssetWriter.canApply(outputSettings:forMediaType:audio) rejects an explicitly empty Data value for AVChannelLayoutKey and returns false, which throws cannotSetupAudioInputs on every audio frame. The recording fails completely — no output file is written.

Steps to reproduce

1. Connect an external USB microphone via Lightning or USB-C adapter (see hardware photo below)
2. Launch the app — microphone must be connected before launch
3. Start a video recording session
4. Recording always fails with cannotSetupAudioInputs

Connecting the microphone after launch works fine because AVCaptureSession initialises audio normally in that case.

Hardware reference

[photo of USB mic connected via Lightning adapter — showing the exact setup that triggers the degraded audio state]

Fix

Three-layer defence in appendAudioSampleBuffers:

1. Omit AVChannelLayoutKey when layout data is absent
Per AVAssetWriterInput docs, AVChannelLayoutKey is only required for channel counts > 2. For ≤2 channels, AVAssetWriter correctly infers mono/stereo from AVNumberOfChannelsKey when the key is absent.

2. Guard zero ASBD values
Fall back to 44100 Hz / 1 ch when mSampleRate or mChannelsPerFrame are zero. Valid non-zero values from a healthy format description are used unchanged — normal recording is completely unaffected.

3. Last-resort canApply retry
If the format-description-derived settings are still rejected, retry with hardcoded 44100 Hz mono AAC and nil format hint. This combination is unconditionally accepted by AVAssetWriter for AAC output.

Testing

Tested with a USB microphone (Lightning adapter) plugged in both before and after launch:

• Video session ✅
• Built-in mic (no external device) ✅

Note: MultitrackMovieRecorder is only involved in video recording. Other session types (photo etc) use a different path and are not affected by this bug.