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/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)