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.

clients/apple/iOS/VoiceCatiOS/AudioSessionManager.swift

@@ -15,6 +15,12 @@ final class AudioSessionManager {
     /// channel count (e.g. when switching stereo → mono) without going through `SessionState`.
     var activeMicStreamId: UInt32?
 
+    /// Set by `SessionState`. Invoked by `IOSAudioRouter` after an audio-config change so the
+    /// voice path (native VPIO vs the core's miniaudio path) can be restarted to match the new
+    /// preset/route when the mic is active. No-op when not in voice. See
+    /// `SessionState.reconcileVoicePath()` and `IOSVoiceProcessingEngine`.
+    var reconcileVoicePath: (() -> Void)?
+
     /// Tracks whether WE activated the session. The session must be active whenever the
     /// AudioEngine is running (for capture OR playback), so it is activated when any audio
     /// needs to play (a remote stream started OR the user joins voice) and only deactivated