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.

docs/voice.md

@@ -215,6 +215,23 @@ Each receiver keeps an **adaptive jitter buffer per ssrc**.
   `AVAudioSession`, then restarts the devices (`vc_audio_restart`) so they reopen against the
   new route — mirroring TeamTalk5's `closeSoundDevices`/`initSoundInputDevice`/
   `initSoundOutputDevice` pattern.
+- **iOS voice processing (AEC/NS/AGC) — native VPIO path.** Real iOS echo cancellation, noise
+  suppression and AGC are provided ONLY by Apple's **Voice-Processing I/O audio unit (VPIO)**,
+  *not* by the `AVAudioSession` mode alone. The core uses miniaudio's plain `RemoteIO` audio
+  units, which never engage VPIO — so `.voiceChat` mode by itself yields no AEC. For VPIO to
+  cancel echo it must own BOTH the mic capture and the remote-audio playback (it subtracts the
+  played-back signal from the mic), so on the AEC presets (Voice Chat / Bluetooth Headset HFP /
+  Wired Headset) the Swift layer runs a native `AVAudioEngine` with
+  `inputNode.setVoiceProcessingEnabled(true)` and the core runs in **external mode**:
+  - **Mic:** the MIC stream is started with `vc_stream_desc.external_feed=1`; the VPIO input tap
+    feeds processed mic PCM via `vc_stream_feed_pcm`. The core skips its hardware capture device
+    (`AudioParams.external_capture`).
+  - **Playback:** `vc_set_external_playback(1)` makes the core skip its hardware playback device;
+    a mixer-timer thread drives decode+mix on a ~20 ms cadence and delivers the FINAL mixed PCM
+    via `vc_set_mixed_output_sink`. The Swift engine renders that through the VPIO output, so
+    VPIO has its echo-cancellation reference signal.
+  The Stereo Mic / Studio / A2DP presets keep the miniaudio path (they want raw / stereo /
+  no-AEC routing that VPIO can't provide — VPIO forces mono).
 - **DSP engine: see §11.** The original plan was `webrtc-audio-processing` (AEC + NS + AGC +
   VAD in one tuned module, BSD-licensed) — but it has no working Windows/MSVC build upstream
   (confirmed via its own issue tracker: GCC-only Meson build, MinGW support unfinished, hard