fix(macos): avoid Bluetooth headset mics in automatic selection

[kitlangton]

Why

Opening a Bluetooth headset's microphone switches AirPods and similar headsets into a low-quality call profile for everything they play. With the microphone set to Automatic, HEX opened the macOS default input, which becomes the headset as soon as it connects. With Keep ready, music sounded bad the whole time HEX was running (#106). With Release when idle, every shortcut chord that briefly matched the dictation key opened the mic and made the audio blip (#88).

What Changes

Automatic selection keeps its existing order (compiled device preferences, then the macOS default) with one change: when that default is a Bluetooth input and the Mac has a built-in microphone, HEX opens the built-in microphone instead.

Inputs present	macOS default	Automatic opens
MacBook Pro Microphone, AirPods Pro	AirPods Pro	MacBook Pro Microphone
MacBook Pro Microphone, USB mic	USB mic	USB mic
AirPods Pro only (for example a Mac mini)	AirPods Pro	AirPods Pro

A headset chosen explicitly in Settings, and the CLI --device override, are still opened as before. Each skip is logged with both device names.

The transport comes from CoreAudio (kAudioDevicePropertyTransportType, Bluetooth or Bluetooth LE versus built-in), resolved from cpal's device UID, because cpal does not report it on macOS.

Scope

Fixes #106 and #88 for Automatic selection. It does not change an explicitly saved device or follow default-device changes while a warm stream is already open.

Verification

cargo fmt --check
cargo clippy --all-targets --all-features -- -D warnings
cargo test
git diff --check

• 492 tests passed, 13 ignored, including the new automatic_input_prefers_built_in_over_a_bluetooth_default selection table.
• A temporary live probe on this Mac (not committed) enumerated real inputs through the new CoreAudio lookup: MacBook Pro Microphone: BuiltIn, BlackHole 2ch: Other, iPhone Continuity mic Other. AirPods were not connected, so the Bluetooth branch has not been exercised against a real headset yet.