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/IOSAudioRouter.swift

@@ -689,9 +689,35 @@ final class IOSAudioRouter: ObservableObject {
         showsA2dpNoAecWarning = (bluetoothMode == .builtInMicBtA2dp)
     }
 
-    /// Whether the current configuration wants Bluetooth A2DP output. Used after session
-    /// activation to clear any lingering speaker override that would pin output to the speaker.
-    var wantsA2dpOutput: Bool { bluetoothMode == .builtInMicBtA2dp }
+    /// Route fallback for the A2DP-output presets (Stereo Mic / Studio / BT Headphones + Mono
+    /// Mic, all `.builtInMicBtA2dp`). These presets deliberately omit `.defaultToSpeaker` (it
+    /// breaks A2DP routing) and skip the `forceSpeaker` override, so when NO external output
+    /// (Bluetooth A2DP / wired / AirPlay) is connected `.playAndRecord` pins output to the quiet
+    /// built-in receiver (earpiece). This routes to the loud built-in speaker instead via a
+    /// post-activation `overrideOutputAudioPort(.speaker)` — the documented "A2DP if connected,
+    /// else speaker" behavior. When an external output IS present we clear the override so A2DP /
+    /// headphones / AirPlay are honored. No-op outside `.builtInMicBtA2dp` mode (other modes pick
+    /// their route via category options). Must be called AFTER the session is active.
+    func applyA2dpSpeakerFallback() {
+        guard bluetoothMode == .builtInMicBtA2dp else { return }
+        let session = AVAudioSession.sharedInstance()
+        // Treat the built-in receiver and speaker as "internal"; anything else (A2DP, headphones,
+        // USB, AirPlay) is an external output we should defer to.
+        let hasExternalOutput = session.currentRoute.outputs.contains {
+            $0.portType != .builtInReceiver && $0.portType != .builtInSpeaker
+        }
+        do {
+            if hasExternalOutput {
+                try session.overrideOutputAudioPort(.none)
+                logger.info("A2DP mode — external output present, clearing speaker override")
+            } else {
+                try session.overrideOutputAudioPort(.speaker)
+                logger.info("A2DP mode — no external output, routing to built-in speaker")
+            }
+        } catch {
+            logger.error("A2DP speaker fallback failed: \(error.localizedDescription)")
+        }
+    }
 
     /// The selected input port object, if any.
     var selectedPort: IOSAudioInputPort? {