fix(apple-client/audio): capture the right channel of a multi-channel mic + diagnostics

The mic uplink handed the host pure digital silence on a multi-channel
interface: AVAudioConverter's N→stereo downmix takes channels 0/1, but a
pro interface puts the mic on ONE higher discrete channel. Fold the input
to a mono bus ourselves instead — pick the mic's channel (or sum all) and
resample that to the encoder's 48 kHz stereo, so the silent 0/1 downmix
never happens.

- New "Microphone channel" setting (macOS): Auto (sum every channel — a
  lone hot mic passes at full level) or pin 1-based channel N. Picker
  appears only for multi-channel devices, driven by the device's input
  channel count.
- Diagnostics that make this class of failure self-naming next session:
  log the actual live capture device + format + fold mode, warn on a
  silent UID fallback, and a one-shot silence tripwire on the EXTRACTED
  signal (WARN on 10 s of zeros, else peak dBFS).
- foldToMono extracted as a pure, unit-tested helper (pin / sum-clamp x
  interleaved / deinterleaved / mono / out-of-range).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/apple/Sources/PunktfunkClient/Session/SessionModel.swift

@@ -338,6 +338,7 @@ final class SessionModel: ObservableObject {
         audio.start(
             speakerUID: defaults.string(forKey: DefaultsKey.speakerUID) ?? "",
             micUID: defaults.string(forKey: DefaultsKey.micUID) ?? "",
+            micChannel: defaults.integer(forKey: DefaultsKey.micChannel),
             micEnabled: defaults.object(forKey: DefaultsKey.micEnabled) as? Bool ?? true)
         self.audio = audio
         // Gamepads: forward GamepadManager's active controller as pad 0 and render the