fix(ios): stereo mic + A2DP output, add vc_audio_restart ABI

Diagnosed by comparing against TeamTalk5 (Client/iTeamTalk), which
achieves stereo mic + A2DP output. Five fixes:

1. configureStereoCapture now calls setPreferredInput +
   setInputDataSource (mirroring TeamTalk5's SoundDevicesModel).
   Previously omitted based on incorrect diagnosis that
   setPreferredInput collapsed A2DP — the real culprit was
   setPreferredInputNumberOfChannels(2), which neither project uses.

2. New C ABI: vc_audio_restart (full stop + re-init, unlike
   suspend/resume which only stop/start). Swift wrapper added.
   The withAudioSuspend wrapper that used it was removed after
   on-device testing showed it killed all audio (including
   VoiceOver) when switching presets — the core's
   set_capture_channels handles engine restart internally.

3. Bluetooth options: Voice Chat preset now includes BOTH
   .allowBluetoothHFP AND .allowBluetoothA2DP (matching TeamTalk5's
   UtilSound.swift:228). Previously HFP-only blocked A2DP headphones.

4. Capture channels now reset when switching stereo→mono via
   selectCaptureChannels/applyPreset. AudioSessionManager tracks
   activeMicStreamId (set by SessionState on join/leave voice).

5. Docs synced: voice.md, tech-stack.md, architecture.md,
   PROGRESS.md. Removed stale setPreferredInputNumberOfChannels(2)
   references.

Verified: ctest --preset dev 21/21 green, iOS client builds.
Stereo mic + A2DP output still needs on-device debugging — the
core recipe is correct but iOS 26 route behavior requires
hands-on testing with a debugger.

core/include/voicecat.h

@@ -399,11 +399,12 @@ VC_API vc_result vc_test_inject_capture(vc_client* c, uint32_t stream_id, const
 /* Set the capture channel count for a local MIC stream (1 = mono, 2 = stereo interleaved).
  * Must be called after vc_stream_start; takes effect on the next AudioEngine restart (e.g. when
  * joining voice, or immediately if the engine is already running — it stops and restarts the
- * capture device with the new channel count). Defaults to 1 (mono). On iOS this lets the Swift
- * AVAudioSession routing layer request stereo built-in mic capture via
- * setPreferredInputNumberOfChannels(2) and then tell the core to open the capture device in
- * stereo. VC_ERR_INVALID_ARG if stream_id is unknown, the stream is not a MIC stream, or
- * channels is not 1 or 2. */
+ * capture device with the new channel count). Defaults to 1 (mono). On iOS the Swift
+ * AVAudioSession routing layer enables stereo built-in mic capture by switching the built-in
+ * mic's data source to the .stereo polar pattern (setPreferredDataSource +
+ * setPreferredPolarPattern(.stereo) + setPreferredInput + setInputDataSource) and then calls
+ * this to tell the core to open the capture device in stereo. VC_ERR_INVALID_ARG if stream_id
+ * is unknown, the stream is not a MIC stream, or channels is not 1 or 2. */
 VC_API vc_result vc_set_capture_channels(vc_client* c, uint32_t stream_id, uint32_t channels);
 
 /* ── Text ─────────────────────────────────────────────────────────────────── */
@@ -484,6 +485,16 @@ VC_API vc_result vc_get_permissions(vc_client* c, vc_permissions* out);
 VC_API vc_result vc_audio_suspend(vc_client* c);
 VC_API vc_result vc_audio_resume(vc_client* c);
 
+/* Full audio engine restart (iOS M6). Unlike vc_audio_suspend/resume which only stop/start
+ * the existing miniaudio devices (leaving them bound to the route that was active when they
+ * were opened), vc_audio_restart() uninitializes and re-initializes the capture and playback
+ * devices so they pick up a new AVAudioSession route. Call this from the Swift layer AFTER
+ * reconfiguring AVAudioSession (setCategory, setPreferredInput, setPreferredPolarPattern, etc.)
+ * so the core's devices reopen against the new route. Mirrors TeamTalk5's
+ * closeSoundDevices()/initSoundInputDevice()/initSoundOutputDevice() reconfiguration pattern.
+ * Safe to call when the engine is not running (it will just start it). */
+VC_API vc_result vc_audio_restart(vc_client* c);
+
 #if defined(__cplusplus)
 } /* extern "C" */
 #endif

core/src/core/client.cpp

@@ -1433,6 +1433,22 @@ vc_result vc_client::audio_resume() {
     return audio_engine_.resume() ? VC_OK : VC_ERR_AUDIO;
 }
 
+vc_result vc_client::audio_restart() {
+    // Full uninit + re-init (not just stop/start like suspend/resume) so the miniaudio
+    // devices reopen against the current AVAudioSession route. Mirrors TeamTalk5's
+    // closeSoundDevices()/initSoundInputDevice()/initSoundOutputDevice() pattern.
+    // IMPORTANT: only restart if the engine was already running — calling
+    // ensure_audio_running() when the engine isn't running would start it prematurely
+    // (opening the capture device / mic without an active mic stream), which on iOS
+    // triggers a route reconfiguration that can collapse the output route.
+    bool was_running = audio_engine_.running();
+    audio_engine_.stop();
+    if (was_running) {
+        ensure_audio_running();
+    }
+    return VC_OK;
+}
+
 vc_result vc_client::get_stream_audio_config(uint32_t user_id, uint32_t stream_id,
                                              vc_audio_config* out) {
     if (state_net_.load(std::memory_order_acquire) != VC_STATE_CONNECTED) return VC_ERR_NOT_CONNECTED;
@@ -1900,5 +1916,6 @@ vc_result vc_client::get_account_list(vc_account_list*) { return VC_ERR_NOT_IMPL
 vc_result vc_client::get_permissions(vc_permissions*) { return VC_ERR_NOT_IMPLEMENTED; }
 vc_result vc_client::audio_suspend() { return VC_ERR_NOT_IMPLEMENTED; }
 vc_result vc_client::audio_resume()  { return VC_ERR_NOT_IMPLEMENTED; }
+vc_result vc_client::audio_restart() { return VC_ERR_NOT_IMPLEMENTED; }
 
 #endif  // VOICECAT_HAS_NET

core/src/core/client.h

@@ -60,6 +60,7 @@ struct vc_client {
                                 vc_remote_stream_state* out);
     vc_result audio_suspend();
     vc_result audio_resume();
+    vc_result audio_restart();
 
     vc_result send_text(vc_text_scope scope, uint32_t target_id, const char* utf8);
 

core/src/voicecat.cpp

@@ -310,4 +310,9 @@ vc_result vc_audio_resume(vc_client* c) {
     return c->audio_resume();
 }
 
+vc_result vc_audio_restart(vc_client* c) {
+    if (c == nullptr) return VC_ERR_INVALID_ARG;
+    return c->audio_restart();
+}
+
 }  // extern "C"