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/Models.swift

@@ -212,9 +212,14 @@ public struct StreamDescriptor: Sendable, Equatable {
     /// nil = default device for this kind.
     public let deviceId: String?
     public let label: String
+    /// When true the caller feeds PCM via `feedPcm` (e.g. the iOS VPIO mic path) and the core
+    /// skips opening a hardware capture device for this stream. Mirrors `vc_stream_desc.external_feed`.
+    public let externalFeed: Bool
 
-    public init(kind: VoiceCatStreamKind, deviceId: String? = nil, label: String) {
+    public init(kind: VoiceCatStreamKind, deviceId: String? = nil, label: String,
+                externalFeed: Bool = false) {
         self.kind = kind; self.deviceId = deviceId; self.label = label
+        self.externalFeed = externalFeed
     }
 }