feat: fail over to an external mic when the MacBook lid is closed

[IchenDEV]

Problem

On a MacBook used in clamshell mode (external display, lid closed), the built-in microphone is physically disconnected in hardware, but macOS still lists it in CoreAudio as an available input device. Utter selected it, recorded silence, and never surfaced an error — the user spoke, saw the overlay, and got no text. This is the docked-at-a-desk case, a primary usage context for a menu-bar dictation app.

Fix

• Lid detection — read AppleClamshellState from the IOPMrootDomain IORegistry entry (ClamshellState). Desktops without a lid report open and are unaffected.
• Device classification — enumerate input devices and flag the built-in mic via kAudioDevicePropertyTransportType (AudioInputDevices). Continuity/iPhone, USB, Bluetooth, display, and aggregate inputs are all treated as external.
• Resolution — a pure AudioInputResolver keeps the user's selected/system-default input when usable, and otherwise falls back to the first available external input; .unavailable when none exists.
• Mid-session failover — while recording, a 0.5s watchdog re-checks the active input. If it becomes unusable (lid closes over the built-in mic, or the device is removed), capture restarts on a fallback input and the session continues; buffers are converted with AVAudioConverter when the new device's format differs.
• Fail fast — when the lid is closed and no external input exists, the session ends with a new localized message and no insertion/clipboard/history side effect.
• The wireless-remote (Xiaomi) capture path is unchanged.

Files

• Added: Sources/Audio/ClamshellState.swift, Sources/Audio/AudioInputDevice.swift, Sources/Audio/AudioInputResolver.swift, Tests/OpenTypeTests/AudioInputResolverTests.swift
• Changed: Sources/Audio/AudioCaptureManager.swift, Sources/App/VoicePipeline+Recording.swift, Sources/Integration/InputSessionCoordinator.swift, both Localizable.strings
• SDLC bundle: docs/sdlc/changes/2026-10-01-clamshell-mic-failover/

Verification

• bash scripts/sdlc-checks.sh — pass
• bash scripts/ci-basic-checks.sh — pass (strings lint + key parity)
• swift test — 808 tests, 18 skipped, 0 failures (run via the Xcode toolchain; the CommandLineTools SDK has no XCTest)
• New resolver/classification/failover tests: 14 cases

Residual risk

• Real hardware failover (engine restart + format conversion) and the no-input message are not exercised on a physical docked MacBook in this environment — a maintainer should run the docked check before merge.
• Bluetooth/Continuity fallback may add latency or lower fidelity.
• A failover seam may drop up to the watchdog interval (≤0.5s) of audio.

The SDLC verification artifact is at Status: pending approval pending human review.