fix(audio): guard against invalid inputNode format after fresh mic permission grant

[thedavidweng]

Summary

Fix -10877 (kAudioUnitErr_InvalidElement) crash that occurs when a user triggers voice input for the first time immediately after granting microphone permission.

Root Cause

After a fresh microphone permission grant, AVAudioEngine.inputNode may temporarily report 0 channels / 0 sampleRate while the macOS audio subsystem reconfigures. The previous code proceeded unconditionally with this invalid format:

AVAudioFormat *hardwareFormat = [inputNode outputFormatForBus:0];
// channelCount=0, sampleRate=0 — no validation

[inputNode installTapOnBus:0 bufferSize:4096 format:hardwareFormat block:...];
[self.audioEngine startAndReturnError:&error]; // throws -10877

AVAudioEngine.start() then fails with -10877 because the Audio Unit has no valid element/connection on bus 0. The user sees this as "pressing the trigger key does nothing" with throwing -10877 flooding the Console log.

Reproduction

1. Fresh install Koe v1.0.14 (build 15)
2. Launch → grant Microphone permission when prompted
3. Immediately press the trigger key (Fn)
4. Console shows multiple throwing -10877 entries

Changes

SPAudioCaptureManager.m

• Add format validation after querying outputFormatForBus:0 — return NO early if channelCount == 0 or sampleRate <= 0, with a descriptive log message

SPAppDelegate.m

• Extract duplicated audio-capture-start code (hold + tap paths) into startAudioCaptureWithRetry
• On first capture failure, retry once after 500ms to allow the audio subsystem time to settle after a permission change
• Retry is session-token-guarded to avoid stale execution if the user triggers a new session during the delay

Behavior

Scenario	Before	After
First use after granting mic permission	-10877 crash, no audio	Retry after 500ms, capture succeeds
Mic permission already granted	Works	Works (no retry needed, first attempt succeeds)
Mic permission denied	Error cue, no explanation	Error cue (unchanged; permission alert is a separate PR)
Bluetooth device reconnect mid-session	Works (existing recovery)	Works (unchanged)

Testing

• All Rust tests pass (cargo test --no-default-features — 44+11+1 tests)
• Modified .m files verified for balanced braces/brackets/parens
• cargo fmt --check / cargo clippy — pass