fix(ios-audio): unify iOS audio onto one always-external AVAudioEngine

The iOS audio path was a hybrid: Voice-Chat-class presets ran a native
VPIO AVAudioEngine (core external) while Stereo/Studio/A2DP presets ran
the core's miniaudio devices. Nearly every "no input / no output / both"
bug lived in the seam between the two paths — the lingering miniaudio
capture unit fighting VPIO, the audioRestart ordering dance, the
route-change "glitching" loop, stereo<->mono stickiness, and
"can't hear anyone". Switching presets/routes mid-call routinely dropped
a direction.

Drive ALL iOS audio through one AVAudioEngine with the core fully
external at all times: setExternalPlayback(1) once at connect, every MIC
stream external_feed=1, mic via vc_stream_feed_pcm, playback via
vc_set_mixed_output_sink (drained by an always-on AVAudioSourceNode so
remote audio plays before joining voice). VPIO + AGC toggle per preset.
Every preset/route/interruption change funnels through one deterministic
Swift-only reconfigure (stop -> apply session config -> rebuild -> start)
— no second path to hand off to, so a change can't drop a direction.

- IOSVoiceProcessingEngine.swift -> IOSAudioEngine: always-on source-node
  playback, conditional mic tap, VPIO/AGC; one rebuild() backing
  startListening/stop/startMic/stopMic/reconfigure/setCaptureChannels.
- IOSAudioRouter: 7 presets -> 4 (Voice Chat / Stereo Mic / Mono Mic /
  Advanced); persisted voiceProcessingEnabled + agcEnabled; setters call
  IOSAudioEngine.reconfigure() instead of audioRestart/reconcileVoicePath.
- AudioSessionManager slimmed; SessionState mic lifecycle collapsed;
  AppState wires external playback + listening at connect, stop at
  disconnect; SettingsView shows 4 presets + Advanced VPIO/AGC toggles.

No core/ABI/test changes — relies on the already-shipped external API
(test_external_pcm, test_external_playback). xcodebuild iOS device Debug
BUILD SUCCEEDED. Updates docs/voice.md §8 and PROGRESS.md.

clients/apple/iOS/VoiceCatiOS/Views/SettingsView.swift

@@ -38,9 +38,9 @@ struct SettingsView: View {
                     .accessibilityLabel("Speaker output")
                     .accessibilityHint("Routes audio to the speaker instead of the earpiece when no headphones are connected.")
 
-                    // Surface the voice-processing state. On the AEC presets the native iOS
+                    // Surface the voice-processing state. On Voice Chat the native iOS
                     // Voice-Processing unit (VPIO) does echo cancellation, noise suppression and
-                    // automatic gain control; the other presets (stereo/studio/A2DP) can't use it.
+                    // automatic gain control; the stereo / mono-mic / A2DP configs can't use it.
                     if router.currentConfigUsesVoiceProcessing {
                         Label("Echo cancellation & noise suppression on (iOS voice processing)",
                               systemImage: "waveform.badge.mic")
@@ -48,18 +48,11 @@ struct SettingsView: View {
                             .foregroundStyle(.secondary)
                             .accessibilityLabel("Echo cancellation and noise suppression are on")
                     } else {
-                        Label("No echo cancellation in this preset (stereo / studio / A2DP)",
+                        Label("No echo cancellation in this configuration (stereo / A2DP / off)",
                               systemImage: "waveform.slash")
                             .font(.caption)
                             .foregroundStyle(.secondary)
-                            .accessibilityLabel("Echo cancellation is off in this preset")
-                    }
-
-                    if !router.hasBluetoothDevice && !router.hasWiredHeadset {
-                        Text("Connect Bluetooth headphones or a wired headset for more presets.")
-                            .font(.caption)
-                            .foregroundStyle(.secondary)
-                            .accessibilityLabel("No external audio device connected")
+                            .accessibilityLabel("Echo cancellation is off in this configuration")
                     }
                 }
 
@@ -127,6 +120,27 @@ struct SettingsView: View {
                         }
                         .accessibilityLabel("Microphone processing mode")
 
+                        // Voice-processing (VPIO) controls — only meaningful on a VPIO-capable
+                        // config (mono + standard + non-A2DP). iOS bundles echo cancellation and
+                        // noise suppression into one master switch (no per-stage toggle); AGC is
+                        // the one sub-stage it lets us control independently.
+                        if router.voiceProcessingAvailable {
+                            Toggle("Voice Processing (AEC + noise suppression)", isOn: Binding(
+                                get: { router.voiceProcessingEnabled },
+                                set: { router.setVoiceProcessingEnabled($0) }
+                            ))
+                            .accessibilityLabel("Voice processing")
+                            .accessibilityHint("Echo cancellation and noise suppression, bundled together by iOS.")
+
+                            if router.voiceProcessingEnabled {
+                                Toggle("Automatic Gain Control", isOn: Binding(
+                                    get: { router.agcEnabled },
+                                    set: { router.setAgcEnabled($0) }
+                                ))
+                                .accessibilityLabel("Automatic gain control")
+                            }
+                        }
+
                         if router.showsRawModeSpeakerWarning {
                             Label(
                                 "Raw mode on speaker — echo risk (no AEC)",