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()

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? {

clients/apple/iOS/VoiceCatiOS/IOSVoiceProcessingEngine.swift

@@ -75,6 +75,14 @@ final class PCMRing {
 
     /// Discard everything buffered — call before (re)starting so stale pre-roll isn't played.
     func reset() { OSMemoryBarrier(); readIdx = writeIdx }
+
+    /// Diagnostics: monotonic total samples written / read since the ring was created. The
+    /// indices are already cumulative, so these are free. Only read them when both threads are
+    /// quiesced (e.g. at teardown after the engine + mixer sink are stopped) — they are not
+    /// synchronized for live cross-thread reads. Lets us tell "core never delivered PCM" (Bug 1
+    /// core path) apart from "PCM arrived but produced no sound" (AVAudioEngine output graph).
+    var debugTotalWritten: UInt64 { writeIdx }
+    var debugTotalRead: UInt64 { readIdx }
 }
 
 /// Native iOS voice-processing audio path (docs/voice.md §8 "iOS voice processing").
@@ -215,7 +223,17 @@ final class IOSVoiceProcessingEngine {
         do {
             try engine.start()
             isRunning = true
-            logger.info("VPIO engine started — inFormat=\(inFormat), captureCh=\(targetCh)")
+            // Diagnostics: capture the negotiated graph formats and the live output route so a
+            // silent-playback report can be triaged (format/rate mismatch vs. routing vs. the
+            // core not delivering PCM — see the ring stats logged in teardown()).
+            let outFmt = engine.outputNode.outputFormat(forBus: 0)
+            let mixFmt = engine.mainMixerNode.outputFormat(forBus: 0)
+            let route = AVAudioSession.sharedInstance().currentRoute.outputs
+                .map { "\($0.portName)[\($0.portType.rawValue)]" }.joined(separator: ", ")
+            logger.info("""
+                VPIO engine started — inFormat=\(inFormat), captureCh=\(targetCh), \
+                outputNode=\(outFmt), mainMixer=\(mixFmt), outputRoute=[\(route)]
+                """)
         } catch {
             logger.error("VPIO engine start failed: \(error.localizedDescription)")
             teardown()
@@ -234,6 +252,10 @@ final class IOSVoiceProcessingEngine {
         client?.setMixedOutputSink(nil, user: nil)
         engine.inputNode.removeTap(onBus: 0)
         if engine.isRunning { engine.stop() }
+        // Diagnostics (threads now quiesced): how much mixed PCM the core delivered into the ring
+        // vs. how much the render thread consumed. written==0 ⇒ the core never delivered (Bug 1
+        // core/lifecycle path); written>0 with no audible output ⇒ the AVAudioEngine output graph.
+        logger.info("VPIO ring stats — written=\(self.ring.debugTotalWritten) read=\(self.ring.debugTotalRead) samples")
         try? engine.inputNode.setVoiceProcessingEnabled(false)
         if let src = sourceNode {
             engine.detach(src)

clients/apple/iOS/VoiceCatiOS/SessionState.swift

@@ -230,13 +230,16 @@ final class SessionState {
         // runs in external mode (no hardware mic/playback). The mic stream is started with
         // externalFeed so the core skips the hardware capture device; setExternalPlayback makes
         // it skip the hardware playback device and deliver the mix to IOSVoiceProcessingEngine.
+        //
+        // ORDER MATTERS: set the external-playback flag now, but defer audioRestart() until
+        // AFTER startStream (below) so the MIC LocalStream — which carries external_feed=true —
+        // already exists when ensure_audio_running() derives external_capture. Restarting before
+        // the stream exists makes the core reopen a hardware capture device that is never dropped
+        // (the announce-result restart early-returns because the engine is already running); that
+        // lingering miniaudio capture unit then fights the AVAudioEngine VPIO unit on the same
+        // .voiceChat session and silences VPIO playback.
         let useVPIO = IOSAudioRouter.shared.currentConfigUsesVoiceProcessing
-        if useVPIO {
-            client.setExternalPlayback(true)
-            client.audioRestart()  // flip any already-running (pre-join) engine into external mode
-        } else {
-            client.setExternalPlayback(false)
-        }
+        client.setExternalPlayback(useVPIO)
 
         let desc = StreamDescriptor(kind: .mic, deviceId: voiceState.currentDeviceId, label: "Mic",
                                     externalFeed: useVPIO)
@@ -259,6 +262,10 @@ final class SessionState {
                 client.setCaptureChannels(streamId: streamId, channels: channels)
             }
             if useVPIO {
+                // The external-feed MIC stream now exists, so restart the core into full
+                // external mode (no hardware capture/playback, mixer-timer only) — mic and
+                // speaker are owned entirely by the VPIO engine, which we start right after.
+                client.audioRestart()
                 IOSVoiceProcessingEngine.shared.start(
                     client: client, micStreamId: streamId, captureChannels: channels)
             }