[wxtc0219]

What this does

Adds an opt-in "Bluetooth Audio Quality" guard that keeps the default input on the built-in microphone, so AirPods stay on the A2DP link instead of falling back to HFP/SCO (which roughly halves output quality and burns headset battery).

When enabled:

• Bluetooth inputs are disabled in FineTune's input pickers (menu bar popup + settings), so the headset mic cannot be selected as the input by accident.
• The built-in mic is re-pinned on device connect, system wake, default-input change, and Bluetooth output sample-rate changes.
• Corrections are rate limited (one write per second, max four in a rolling ten-second window) and fall back to a single delayed retry, so a stubborn app can't put the guard into a write loop.
• A hint appears when a Bluetooth output reports below 44.1 kHz, which is how SCO/call mode shows up.
• The status line reports what the guard is doing (pinned / contention / unavailable).

Off by default. When disabled, none of the existing input-lock paths change behaviour.

Why the existing input lock doesn't cover this

Lock Input Device (from #29) protects a user-chosen input and correctly steps aside for priority rules and deliberate changes (#428). That means:

• A user who locked, say, a USB mic still has Bluetooth fallbacks applied on top of it.
• Auto-switching on connect can still move the default input before the lock reacts ([BUG]Lock Input Device #459 reports this on some machines).
• Nothing stops the user from selecting the headset mic manually.

This PR adds a separate, stricter mode for the one case where the headset must never become the default input. It intentionally does not claim to disable HFP/SCO: macOS has no public API to make one device's input unavailable to other apps, so if a call app opens the headset mic directly, the guard can only report it.

Testing

• New BluetoothInputGuardPolicyTests covers the selection rules: built-in preference, Bluetooth never chosen (even when it claims builtIn), no built-in mic → no target, disabled → unrestricted, A2DP rates not flagged, SCO rates flagged, unknown rate not flagged.
• Manual on an M-series Mac with AirPods Pro: switching the default input to another device with SwitchAudioSource/System Settings is reverted within ~1 s; after disabling the guard, the same switch sticks. Wake and reconnect paths behave the same.

Notes

• Settings key bluetoothInputProtection, default false, so users opt in.
• No audio is captured; the guard only reads device metadata and sets the default input.
• Happy to rename the setting or restructure the UI if you'd prefer it inside the existing input-lock section instead of its own section.