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/IOSAudioRouter.swift

@@ -307,6 +307,14 @@ final class IOSAudioRouter: ObservableObject {
         return .custom
     }
 
+    /// Whether the current configuration should use the native iOS Voice-Processing path (VPIO:
+    /// real AEC/NS/AGC via `IOSVoiceProcessingEngine`). True exactly when `applyConfiguration`
+    /// selects the `.voiceChat` AVAudioSession mode — mono + standard processing + not A2DP
+    /// (A2DP / stereo / raw modes can't use VPIO, so they keep the core's miniaudio path).
+    var currentConfigUsesVoiceProcessing: Bool {
+        captureChannels == .mono && micMode == .standard && bluetoothMode != .builtInMicBtA2dp
+    }
+
     // MARK: - Apply configuration
 
     /// Apply the full audio configuration to AVAudioSession. Call this before the core
@@ -573,6 +581,8 @@ final class IOSAudioRouter: ObservableObject {
         savePreferences()
         applyConfiguration()
         refreshRoutes()
+        // VPIO class or route may have changed — restart the voice path if mic is active.
+        AudioSessionManager.shared.reconcileVoicePath?()
     }
 
     func setForceSpeaker(_ on: Bool) {
@@ -580,6 +590,8 @@ final class IOSAudioRouter: ObservableObject {
         savePreferences()
         applyConfiguration()
         refreshRoutes()
+        // Route changed under a possibly-running VPIO engine — reconcile if mic is active.
+        AudioSessionManager.shared.reconcileVoicePath?()
     }
 
     func selectMicMode(_ mode: MicMode) {
@@ -587,6 +599,8 @@ final class IOSAudioRouter: ObservableObject {
         savePreferences()
         applyConfiguration()
         updateWarnings()
+        // Standard↔Raw flips the VPIO class — reconcile if mic is active.
+        AudioSessionManager.shared.reconcileVoicePath?()
     }
 
     func selectCaptureChannels(_ channels: CaptureChannels) {
@@ -603,6 +617,8 @@ final class IOSAudioRouter: ObservableObject {
         // route), then capture — avoiding the race where stereo capture activation drops
         // A2DP before the playback device has a chance to claim the route.
         _ = AudioSessionManager.shared.client?.audioRestart()
+        // Mono↔stereo flips the VPIO class (stereo can't use VPIO) — reconcile if mic is active.
+        AudioSessionManager.shared.reconcileVoicePath?()
     }
 
     // MARK: - Presets
@@ -617,6 +633,10 @@ final class IOSAudioRouter: ObservableObject {
         micMode = preset.micMode
         captureChannels = preset.captureChannels
 
+        // Voice Chat is a phone-call experience — default to the loud speaker so output doesn't
+        // land on the quiet earpiece (receiver). Still yields to connected BT/wired output.
+        if preset == .voiceChat { forceSpeaker = true }
+
         if preset.usesBuiltInMic {
             // Find the built-in mic port from available inputs and select it.
             let session = AVAudioSession.sharedInstance()
@@ -648,6 +668,9 @@ final class IOSAudioRouter: ObservableObject {
         // count is stored. Playback opens first (commits A2DP route), then capture.
         _ = AudioSessionManager.shared.client?.audioRestart()
         refreshRoutes()
+        // The preset may have flipped the VPIO class (and/or the route) — restart the voice path
+        // if the mic is active so AEC/NS engage (or disengage) to match the new preset.
+        AudioSessionManager.shared.reconcileVoicePath?()
         logger.info("applyPreset — \(preset.rawValue)")
     }