feat(ios): real echo cancellation/NR via native Voice-Processing engine

iOS "voice chat" had echo and no noise suppression: real iOS AEC/NS/AGC
come only from Apple's Voice-Processing I/O unit (VPIO), but the core
plays/captures via miniaudio's plain RemoteIO units, so .voiceChat mode
alone never engaged AEC.

Core (ABI PATCH 1->2):
- vc_set_mixed_output_sink + vc_set_external_playback. In external mode the
  AudioEngine opens no hardware playback device; a mixer-timer thread drives
  on_playback (decode+mix) on a ~20ms cadence and ships the final mix to the
  sink. start() also skips the hardware capture device when the MIC stream is
  external_feed (AudioParams.external_capture).
- New white-box test test_external_playback (drives the timer with no hw).

iOS/Swift:
- StreamDescriptor.externalFeed; VoiceCatClient.setMixedOutputSink /
  setExternalPlayback wrappers.
- IOSVoiceProcessingEngine: AVAudioEngine + setVoiceProcessingEnabled; mic
  tap -> feedPcm, mixed-sink lock-free ring -> AVAudioSourceNode (both share
  the VPIO unit so AEC has its reference signal).
- IOSAudioRouter.currentConfigUsesVoiceProcessing scopes VPIO to the AEC
  presets; SessionState join/leave + reconcileVoicePath() switch paths;
  Voice Chat defaults to speaker; Settings surfaces AEC/NS state.

Known: pending on-device verification; a few bugs to fix afterward.

clients/apple/iOS/VoiceCatiOS/Views/SettingsView.swift

@@ -30,6 +30,23 @@ struct SettingsView: View {
                     .accessibilityLabel("Speaker output")
                     .accessibilityHint("Routes audio to the speaker instead of the earpiece when no headphones are connected.")
 
+                    // Surface the voice-processing state. On the AEC presets the native iOS
+                    // Voice-Processing unit (VPIO) does echo cancellation, noise suppression and
+                    // automatic gain control; the other presets (stereo/studio/A2DP) can't use it.
+                    if router.currentConfigUsesVoiceProcessing {
+                        Label("Echo cancellation & noise suppression on (iOS voice processing)",
+                              systemImage: "waveform.badge.mic")
+                            .font(.caption)
+                            .foregroundStyle(.secondary)
+                            .accessibilityLabel("Echo cancellation and noise suppression are on")
+                    } else {
+                        Label("No echo cancellation in this preset (stereo / studio / A2DP)",
+                              systemImage: "waveform.slash")
+                            .font(.caption)
+                            .foregroundStyle(.secondary)
+                            .accessibilityLabel("Echo cancellation is off in this preset")
+                    }
+
                     if !router.hasBluetoothDevice && !router.hasWiredHeadset {
                         Text("Connect Bluetooth headphones or a wired headset for more presets.")
                             .font(.caption)