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/Sources/VoiceCatCore/VoiceCatClient.swift

@@ -41,6 +41,10 @@ import Foundation
 /// `VcPcmSinkCallback` delegate.
 public typealias VoiceCatPcmSinkCallback = vc_pcm_sink_cb
 
+/// Swift-idiomatic alias for the C `vc_mixed_output_cb` function-pointer type from `voicecat.h`
+/// — the external mixed-output sink used by the iOS VPIO path (see `setMixedOutputSink`).
+public typealias VoiceCatMixedOutputCallback = vc_mixed_output_cb
+
 /// The Swift wrapper around `vc_client*`. Owns the native handle for its entire lifetime;
 /// `deinit` destroys it. Events and level meters are delivered on the main queue via the
 /// `onEvent` / `onLevel` closures.
@@ -292,6 +296,7 @@ public final class VoiceCatClient {
         desc.kind = descriptor.kind.cValue
         desc.device_id = deviceIdPtr.map { UnsafePointer($0) }
         desc.label = UnsafePointer(labelPtr)
+        desc.external_feed = descriptor.externalFeed ? 1 : 0
 
         let r = vc_stream_start(handle, &desc, &streamId)
         return (VoiceCatResult(r), streamId)
@@ -355,6 +360,26 @@ public final class VoiceCatClient {
         VoiceCatResult(vc_set_pcm_sink(handle, cb, user))
     }
 
+    /// External mixed-output sink (iOS VPIO) — receives the FINAL mixed remote audio as int16
+    /// PCM on the core's mixer-timer thread when external playback is enabled. The Swift VPIO
+    /// renderer copies this into its ring and plays it through the voice-processing output so
+    /// echo cancellation has its reference signal. Pass `nil` to disable. Mirrors
+    /// `vc_set_mixed_output_sink`. The callback MUST NOT block or allocate.
+    @discardableResult
+    public func setMixedOutputSink(_ cb: VoiceCatMixedOutputCallback?,
+                                   user: UnsafeMutableRawPointer?) -> VoiceCatResult {
+        VoiceCatResult(vc_set_mixed_output_sink(handle, cb, user))
+    }
+
+    /// Enable/disable external-playback mode (iOS VPIO). When enabled, the core opens NO hardware
+    /// playback device; it drives decode+mix on a timer and delivers the final mix via
+    /// `setMixedOutputSink`. Apply before the engine starts, or follow with `audioRestart()` to
+    /// apply to a running engine. Mirrors `vc_set_external_playback`.
+    @discardableResult
+    public func setExternalPlayback(_ enabled: Bool) -> VoiceCatResult {
+        VoiceCatResult(vc_set_external_playback(handle, enabled ? 1 : 0))
+    }
+
     @discardableResult
     public func setInputMode(_ mode: VoiceCatInputMode) -> VoiceCatResult {
         VoiceCatResult(vc_set_input_mode(handle, mode.cValue))