feat(ios): real echo cancellation/NR via native Voice-Processing engine

iOS "voice chat" had echo and no noise suppression: real iOS AEC/NS/AGC
come only from Apple's Voice-Processing I/O unit (VPIO), but the core
plays/captures via miniaudio's plain RemoteIO units, so .voiceChat mode
alone never engaged AEC.

Core (ABI PATCH 1->2):
- vc_set_mixed_output_sink + vc_set_external_playback. In external mode the
  AudioEngine opens no hardware playback device; a mixer-timer thread drives
  on_playback (decode+mix) on a ~20ms cadence and ships the final mix to the
  sink. start() also skips the hardware capture device when the MIC stream is
  external_feed (AudioParams.external_capture).
- New white-box test test_external_playback (drives the timer with no hw).

iOS/Swift:
- StreamDescriptor.externalFeed; VoiceCatClient.setMixedOutputSink /
  setExternalPlayback wrappers.
- IOSVoiceProcessingEngine: AVAudioEngine + setVoiceProcessingEnabled; mic
  tap -> feedPcm, mixed-sink lock-free ring -> AVAudioSourceNode (both share
  the VPIO unit so AEC has its reference signal).
- IOSAudioRouter.currentConfigUsesVoiceProcessing scopes VPIO to the AEC
  presets; SessionState join/leave + reconcileVoicePath() switch paths;
  Voice Chat defaults to speaker; Settings surfaces AEC/NS state.

Known: pending on-device verification; a few bugs to fix afterward.

core/src/voicecat.cpp

@@ -158,6 +158,16 @@ vc_result vc_set_pcm_sink(vc_client* c, vc_pcm_sink_cb cb, void* user) {
     return c->set_pcm_sink(cb, user);
 }
 
+vc_result vc_set_mixed_output_sink(vc_client* c, vc_mixed_output_cb cb, void* user) {
+    if (c == nullptr) return VC_ERR_INVALID_ARG;
+    return c->set_mixed_output_sink(cb, user);
+}
+
+vc_result vc_set_external_playback(vc_client* c, int enable) {
+    if (c == nullptr) return VC_ERR_INVALID_ARG;
+    return c->set_external_playback(enable != 0);
+}
+
 vc_result vc_set_capture_channels(vc_client* c, uint32_t stream_id, uint32_t channels) {
     if (c == nullptr) return VC_ERR_INVALID_ARG;
     return c->set_capture_channels(stream_id, channels);