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.

PROGRESS.md

@@ -75,8 +75,42 @@ up instantly. Newest status at the top.
   is now correct. If voice still fails, watch the server's rate-limited `[media] dropped frames —
   unmapped-endpoint=…` line (NAT source-port rewrite would be the next suspect).
 
-- **Awaiting on-device verification (2026-06-22):** **iOS real echo cancellation / noise suppression
-  via native VPIO.** Root cause of "voice chat doesn't sound like a call" (echo + no NR): real iOS
+- **Done (2026-06-23, Swift-only — no core/ABI change; awaiting on-device verification):** **iOS audio
+  stack unified — one always-external `AVAudioEngine`, miniaudio dropped on iOS.** The iOS audio path was
+  a fragile 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 bug lived in the seam
+  (lingering miniaudio capture unit fighting VPIO, the `audioRestart` ordering dance, the route-change
+  "glitching" loop, stereo↔mono stickiness, "can't hear anyone"), and switching presets/routes mid-call
+  routinely dropped input, output, or both. **Fix: drive *all* iOS audio through one `AVAudioEngine` with
+  the core fully external at all times** — `vc_set_external_playback(1)` once at connect, every MIC stream
+  `external_feed=1`, mic via `vc_stream_feed_pcm`, playback via `vc_set_mixed_output_sink`.
+  - `IOSVoiceProcessingEngine.swift` → **`IOSAudioEngine`** (same file): always-on `AVAudioSourceNode`
+    playback (runs whenever connected, so remote audio plays before you join voice); conditional mic tap;
+    VPIO + AGC toggled per config. One private `rebuild()` (stop → set VPIO → install tap → start) backs
+    `startListening`/`stop`/`startMic`/`stopMic`/`reconfigure`/`setCaptureChannels`. Kept the `PCMRing`
+    and ring-stats diagnostics.
+  - `IOSAudioRouter`: presets cut from seven to **four** — Voice Chat (VPIO mono, system output),
+    Stereo Mic / Mono Mic (internal built-in mic regardless of output, A2DP-capable, no VPIO), Advanced
+    (manual). New persisted `voiceProcessingEnabled` (master AEC+NS) + `agcEnabled`; setters now call
+    `IOSAudioEngine.reconfigure()` instead of `client.audioRestart()` + `reconcileVoicePath`. Kept the
+    proven AVAudioSession recipes (category/mode/options, stereo capsule, `applyA2dpSpeakerFallback`).
+  - `AudioSessionManager` slimmed (drops `client`/`activeMicStreamId`/`reconcileVoicePath`; adds
+    `isActive`); interruption-end & device-change now `reconfigure()` the engine. `SessionState`
+    `doStartMicStream`/`stopMicStream` collapsed to start-stream + `startMic`/`stopMic` (no
+    `setExternalPlayback`/`audioRestart` toggling); `reconcileVoicePath` deleted. `AppState` sets external
+    playback + `startListening` at connect, `stop()` at disconnect. `SettingsView` → four presets +
+    Advanced VPIO/AGC toggles.
+  - **No core/ABI/test change** — relies on the already-shipped `vc_set_external_playback` /
+    `external_feed` / `vc_set_mixed_output_sink` / `vc_stream_feed_pcm` path (`test_external_pcm`,
+    `test_external_playback`). `xcodebuild` iOS device Debug **BUILD SUCCEEDED**. **Rebuild the
+    xcframework is NOT required** (no new symbols).
+  - **Next (user, on device):** two iPhones in a channel — verify BOTH directions survive every
+    transition and are never silent unless intended: Voice Chat (no echo, NR), listen-only before joining,
+    join↔leave repeatedly, switch Voice Chat↔Stereo↔Mono↔Advanced *while in voice*, A2DP connect/unplug,
+    wired connect/unplug, phone-call interruption + resume, screen-audio share.
+
+- **Superseded by the 2026-06-23 unification above (2026-06-22):** **iOS real echo cancellation / noise
+  suppression via native VPIO.** Root cause of "voice chat doesn't sound like a call" (echo + no NR): real iOS
   AEC/NS/AGC come only from Apple's Voice-Processing I/O unit (VPIO), but the core uses miniaudio's
   plain RemoteIO units — so `.voiceChat` mode alone never engaged AEC. Fix moves both mic capture and
   playback to a native Swift `AVAudioEngine` (`setVoiceProcessingEnabled`) on the AEC presets, with the