fix(ios-audio): unify iOS audio onto one always-external AVAudioEngine

The iOS audio path was a hybrid: Voice-Chat-class presets ran a native
VPIO AVAudioEngine (core external) while Stereo/Studio/A2DP presets ran
the core's miniaudio devices. Nearly every "no input / no output / both"
bug lived in the seam between the two paths — the lingering miniaudio
capture unit fighting VPIO, the audioRestart ordering dance, the
route-change "glitching" loop, stereo<->mono stickiness, and
"can't hear anyone". Switching presets/routes mid-call routinely dropped
a direction.

Drive ALL iOS audio through one AVAudioEngine with the core fully
external at all times: setExternalPlayback(1) once at connect, every MIC
stream external_feed=1, mic via vc_stream_feed_pcm, playback via
vc_set_mixed_output_sink (drained by an always-on AVAudioSourceNode so
remote audio plays before joining voice). VPIO + AGC toggle per preset.
Every preset/route/interruption change funnels through one deterministic
Swift-only reconfigure (stop -> apply session config -> rebuild -> start)
— no second path to hand off to, so a change can't drop a direction.

- IOSVoiceProcessingEngine.swift -> IOSAudioEngine: always-on source-node
  playback, conditional mic tap, VPIO/AGC; one rebuild() backing
  startListening/stop/startMic/stopMic/reconfigure/setCaptureChannels.
- IOSAudioRouter: 7 presets -> 4 (Voice Chat / Stereo Mic / Mono Mic /
  Advanced); persisted voiceProcessingEnabled + agcEnabled; setters call
  IOSAudioEngine.reconfigure() instead of audioRestart/reconcileVoicePath.
- AudioSessionManager slimmed; SessionState mic lifecycle collapsed;
  AppState wires external playback + listening at connect, stop at
  disconnect; SettingsView shows 4 presets + Advanced VPIO/AGC toggles.

No core/ABI/test changes — relies on the already-shipped external API
(test_external_pcm, test_external_playback). xcodebuild iOS device Debug
BUILD SUCCEEDED. Updates docs/voice.md §8 and PROGRESS.md.

clients/apple/iOS/VoiceCatiOS/AudioSessionManager.swift

@@ -8,19 +8,6 @@ private let logger = Logger(subsystem: "cat.voice.VoiceCatiOS", category: "Audio
 final class AudioSessionManager {
     static let shared = AudioSessionManager()
 
-    weak var client: VoiceCatClient?
-
-    /// The stream ID of the currently active local MIC stream, if any. Set by `SessionState`
-    /// when the user joins/leaves voice so `IOSAudioRouter` can reset the core's capture
-    /// channel count (e.g. when switching stereo → mono) without going through `SessionState`.
-    var activeMicStreamId: UInt32?
-
-    /// Set by `SessionState`. Invoked by `IOSAudioRouter` after an audio-config change so the
-    /// voice path (native VPIO vs the core's miniaudio path) can be restarted to match the new
-    /// preset/route when the mic is active. No-op when not in voice. See
-    /// `SessionState.reconcileVoicePath()` and `IOSVoiceProcessingEngine`.
-    var reconcileVoicePath: (() -> Void)?
-
     /// Tracks whether WE activated the session. The session must be active whenever the
     /// AudioEngine is running (for capture OR playback), so it is activated when any audio
     /// needs to play (a remote stream started OR the user joins voice) and only deactivated
@@ -28,6 +15,10 @@ final class AudioSessionManager {
     /// want to hear remote audio.
     private var isSessionActive = false
 
+    /// Whether the AVAudioSession is currently active (we activated it). Read by `IOSAudioRouter`
+    /// to decide whether the post-activation A2DP speaker fallback can be applied.
+    var isActive: Bool { isSessionActive }
+
     func configure() {
         // Load stored audio routing preferences and apply them before any audio session
         // activation. IOSAudioRouter drives all iOS audio route selection via AVAudioSession;
@@ -114,18 +105,22 @@ final class AudioSessionManager {
 
         switch type {
         case .began:
+            // The system stops our AVAudioEngine and deactivates the session. Nothing to tear
+            // down — `IOSAudioEngine` rebuilds on resume.
             logger.info("interruption began — session suspended by system")
-            isSessionActive = false  // system deactivated us
-            client?.audioSuspend()
+            isSessionActive = false
         case .ended:
             let optionsValue = info[AVAudioSessionInterruptionOptionKey] as? UInt ?? 0
             let options = AVAudioSession.InterruptionOptions(rawValue: optionsValue)
             if options.contains(.shouldResume) {
                 do {
+                    IOSAudioRouter.shared.applyConfiguration()
                     try AVAudioSession.sharedInstance().setActive(true)
                     isSessionActive = true
-                    logger.info("interruption ended — session reactivated")
-                    client?.audioResume()
+                    IOSAudioRouter.shared.applyA2dpSpeakerFallback()
+                    // Rebuild the engine graph against the restored route (both directions).
+                    IOSAudioEngine.shared.reconfigure()
+                    logger.info("interruption ended — session reactivated, engine rebuilt")
                 } catch {
                     logger.error("interruption ended — reactivation failed: \(error.localizedDescription)")
                 }
@@ -162,6 +157,9 @@ final class AudioSessionManager {
             // the loud speaker (not the earpiece), and a replug should hand output back to A2DP.
             if isSessionActive {
                 IOSAudioRouter.shared.applyA2dpSpeakerFallback()
+                // Rebind the engine (both directions) to the new route. The engine owns the route
+                // now, so this is the single thing that re-establishes audio after a device change.
+                IOSAudioEngine.shared.reconfigure()
             }
         }