Recover microphone capture when the device disappears mid-call

[MaxHeimbrock]

Stacked on #308. Makes the SDK survive the active microphone disappearing mid-call — e.g. a Bluetooth headset disconnecting — instead of going permanently silent.

Behavior

• Detection (in CaptureLoop): the mic clip's position counter advances continuously while a device is alive — even in silence — so a counter that hasn't moved for 1 s, or Microphone.IsRecording going false, means the device is gone. No false positives from a quiet room.
• Recovery: end the dead device, then retry once per second until a device is available — preferring the original device if it reappears, otherwise falling back to the system default microphone. The normal start path re-runs the pre-roll, so the new device's rate and fragmentation are re-measured (recovering onto or off the macOS Bluetooth-HFP case works transparently).
• The published track is unaffected throughout: the native source's format is fixed (48 kHz mono, Fix microphone capture: device-true source format + fragment-aware clip reading #308) and captured audio is resampled to it — no source recreation, no republish, no renegotiation. Subscribers just hear a gap until a device is acquired.

Also included, since it is the same mechanism app-initiated: an internal MicrophoneSource.SwitchDevice(deviceName) (the manual primitive, kept non-public until a device-picker API is designed) and a public DeviceName getter.

Robustness

• Each CaptureLoop carries a generation token and retires itself when a newer capture (restart, switch, or recovery) supersedes it — rapid transitions can't leave two loops reading different clips.
• The start sequence pins its device name locally, so a concurrent change can't mix devices mid-start.
• Recovery suspends while the app is backgrounded, so it can't fight the iOS pause/resume handling; the resume path takes over.

Verification

• Runtime + Meet sample compile clean.
• To validate on hardware: publish from the Meet sample with the Bluetooth headset mic, then turn the headset off mid-call. Expect within ~1–2 s:

  MicrophoneSource: device 'MDR-1000X' stopped delivering audio; attempting recovery
  MicrophoneSource device='(default)' clip=...
  MicrophoneSource: recovered capture on device '(default)'
  

  and the receiver hears the built-in mic after a short gap. Turning the headset back on and off again should keep working.

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