fix(ios): VPIO silent playback + quiet speaker in stereo/studio presets

Two on-device bugs in the native iOS Voice-Processing path (Swift-only;
no core/ABI change).

1. Voice Chat (VPIO) silent playback: doStartMicStream() called
   audioRestart() BEFORE startStream, so when the engine was already
   running (a remote stream had started it) it reopened with
   external_capture=false and opened a hardware miniaudio capture device.
   The announce-result restart then early-returns (engine already running)
   so that device was never dropped and fought the AVAudioEngine VPIO unit,
   silencing playback. Now: setExternalPlayback first, then startStream
   (stores external_feed synchronously), THEN audioRestart() — the core
   reopens in full external mode (no hardware devices). Added VPIO
   diagnostics: graph/route formats at start, ring written/read totals at
   teardown.

2. Stereo Mic / Studio quiet earpiece: the .builtInMicBtA2dp presets omit
   .defaultToSpeaker (it breaks A2DP) and skip forceSpeaker, so with no
   Bluetooth connected output pinned to the quiet receiver. New
   IOSAudioRouter.applyA2dpSpeakerFallback() overrides to the built-in
   speaker when no external (A2DP/wired/AirPlay) output is present and
   clears the override when one is — called after activation and on
   device-change route changes.

clients/apple/iOS/VoiceCatiOS/AudioSessionManager.swift

@@ -56,12 +56,10 @@ final class AudioSessionManager {
         let session = AVAudioSession.sharedInstance()
         try session.setActive(true, options: [])
         isSessionActive = true
-        // For the A2DP output presets, make sure output isn't pinned to the built-in speaker.
-        // A2DP routing in .playAndRecord is fragile; clearing any speaker override after the
-        // session is live nudges iOS to honor the Bluetooth output route.
-        if IOSAudioRouter.shared.wantsA2dpOutput {
-            try? session.overrideOutputAudioPort(.none)
-        }
+        // For the A2DP output presets, pick the right output once the session is live: defer to
+        // a connected A2DP/wired/AirPlay route, but fall back to the loud built-in speaker (not
+        // the quiet earpiece) when nothing external is connected. See applyA2dpSpeakerFallback().
+        IOSAudioRouter.shared.applyA2dpSpeakerFallback()
         let route = AVAudioSession.sharedInstance().currentRoute
         let outputNames = route.outputs.map { $0.portName }.joined(separator: ", ")
         let inputNames = route.inputs.map { $0.portName }.joined(separator: ", ")
@@ -160,6 +158,11 @@ final class AudioSessionManager {
         if reason == .oldDeviceUnavailable || reason == .newDeviceAvailable {
             logger.info("routeChange — external device change, re-applying config")
             IOSAudioRouter.shared.applyConfiguration()
+            // Re-evaluate the A2DP-mode speaker fallback: a Bluetooth unplug should drop us onto
+            // the loud speaker (not the earpiece), and a replug should hand output back to A2DP.
+            if isSessionActive {
+                IOSAudioRouter.shared.applyA2dpSpeakerFallback()
+            }
         }
 
         IOSAudioRouter.shared.refreshRoutes()