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.

tests/CMakeLists.txt

@@ -69,6 +69,14 @@ if(VOICECAT_USE_VCPKG_DEPS)
     target_include_directories(test_plc_cap PRIVATE ${VC_TEST_INTERNAL_INCLUDES})
     add_test(NAME plc_cap COMMAND test_plc_cap)
 
+    # External playback (iOS VPIO): the mixer-timer thread drives decode+mix with NO hardware
+    # device and delivers the final mix to the mixed-output sink. White-box AudioEngine test.
+    add_executable(test_external_playback test_external_playback.cpp)
+    target_link_libraries(test_external_playback PRIVATE voicecat::voicecat)
+    target_compile_features(test_external_playback PRIVATE cxx_std_20)
+    target_include_directories(test_external_playback PRIVATE ${VC_TEST_INTERNAL_INCLUDES})
+    add_test(NAME external_playback COMMAND test_external_playback)
+
     # M2 exit criterion: two headless clients relay encrypted Opus frames via the SFU.
     add_executable(test_m2_voice test_m2_voice.cpp)
     target_link_libraries(test_m2_voice PRIVATE voicecat::server)