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/PunktfunkKit/Support/DefaultsKeys.swift

@@ -24,6 +24,12 @@ public enum DefaultsKey {
     public static let micEnabled = "punktfunk.micEnabled"
     public static let speakerUID = "punktfunk.speakerUID"
     public static let micUID = "punktfunk.micUID"
+    /// macOS: which input channel of the chosen mic device feeds the host. 0 = "Auto" (sum every
+    /// channel to mono — a mic on a single input of a multi-channel interface passes at full
+    /// level); n≥1 pins 1-based input channel n. Multi-channel interfaces expose the mic on ONE
+    /// discrete channel, and the default N→stereo downmix grabs channels 0/1 (silence when the mic
+    /// is higher up), so we fold to mono ourselves. Only meaningful for multi-channel devices.
+    public static let micChannel = "punktfunk.micChannel"
     public static let presenter = "punktfunk.presenter"
     /// Request a 10-bit BT.2020 PQ (HDR10) stream. On by default; only takes effect when the host
     /// has HDR content AND this display supports HDR — otherwise the stream stays 8-bit SDR.