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/Settings/SettingsView+Sections.swift

@@ -208,6 +208,17 @@ extension SettingsView {
                 }
             }
             .disabled(!micEnabled)
+            // Multi-channel interfaces only: the mic sits on ONE discrete input, so let the user
+            // pick it. Auto sums every channel (a lone hot mic still passes at full level).
+            if micChannelCount > 1 {
+                Picker("Microphone channel", selection: $micChannel) {
+                    Text("Auto (all channels)").tag(0)
+                    ForEach(1...micChannelCount, id: \.self) { ch in
+                        Text("Channel \(ch)").tag(ch)
+                    }
+                }
+                .disabled(!micEnabled)
+            }
             #endif
         } header: {
             Text("Audio")