Phase 2.5: extract AudioCapture protocol seam (final I/O seam)

[initcore0]

Sixth and final I/O service seam (after #44 #45 #46 #47 #48). AudioRecorder is the largest and most fragile service (~720 LOC AVAudioEngine + AVAudioConverter + CoreAudio — the file the live-chunks crash lived in), so this was done carefully and adversarially reviewed.

Changes

• AudioCapture protocol (autoGainEnabled, onStateChanged/onLevelChanged, selectDevice, start, startStreaming, startStreamingOnSilence, stop) — AudioRecorder conforms.
• RecorderState nested enum → top-level, Equatable, in OpenWhispCore.
• Removed the dead weak var appState back-ref + init(appState:) → override init() (it was assigned in init and never read).
• Protocol-extension convenience overloads startStreamingOnSilence(onChunk:) and stop() forward the recorder's exact VAD defaults (0.75/0.35/12.0/0.018) / nil completion, so AppState's call sites are unchanged — only those overloads match the calls through the protocol type (verified unambiguous).
• The CoreAudio AudioDevice enumeration stays concrete; SettingsView keeps using it directly — correct platform boundary.
• The entire audio body (installTap, converter, the 16kHz Float32 non-interleaved target = the crash fix, chunk rotation, RMS/auto-gain, teardown) is byte-identical; the diff is only the class decl, the moved enum, and the dead ref.
• Adversarial review confirmed exact equivalence on: dead-ref, overload resolution + matching defaults, Equatable addition, trailing-closure binding, device boundary, untouched audio body.
• Tests: +5 with a FakeAudioCapture double pinning the convenience defaults. 117 → 122.
• ROADMAP: all six I/O seams now extracted (status note added).

swift test 122/122 · ./build.sh clean (full app build).

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