feat(ios): audio overhaul, Join/Leave Voice, channel-id sync fix, stereo mic capture

Three iOS client problems fixed plus a new core stereo-mic capture ABI:

1. Channel-id sync bug (mic button permanently dimmed): SessionState never
   synced currentChannelId from the self user's channelId on connect, so the
   mic button (gated on currentChannelId == 0) stayed dimmed. Added
   syncSelfChannel() (mirrors macOS MainWindowController.swift:461,491,522);
   called from init/.channelList/.userJoined/.userLeft/.userUpdated/.joinResult.
   Added applyServerMuteState() + serverMuted/serverDeafened to VoiceState.

2. Join/Leave Voice button: replaced icon-only mic toggle with explicit
   text button (parity with macOS). Mute/deafen disable when not in voice.

3. IOSAudioRouter.swift (new): full AVAudioSession routing layer — input
   port selection, built-in mic orientation/polar patterns, Bluetooth
   HFP/A2DP/Off modes, Standard/Raw mic processing, stereo capture, AirPlay,
   UserDefaults persistence. AudioSessionManager delegates to it.

4. Core stereo-mic capture (append-only ABI): vc_set_capture_channels()
   lets the core open the mic device in stereo (2-ch interleaved). LocalStream
   gains capture_channels; ensure_audio_running reads it; audio_engine.cpp
   capture_accum_ + on_capture updated to channel-aware accumulation. Test
   test_stereo_mic_capture (headless, L!=R stereo round-trip). Swift wrapper
   VoiceCatClient.setCaptureChannels.

5. Settings UI rework: AVAudioSession-derived input/output tree replaces
   miniaudio device picker.

6. iOS deployment target raised to 18.0 (Package.swift + project.pbxproj).
   swift-tools-version 6.0 with swiftLanguageModes .v5.

Docs: tech-stack.md, architecture.md, voice.md, roadmap.md, building.md
updated; stale 'vc_audio_suspend/resume deferred' claims corrected.

Verified: ctest --preset dev 21/21 green; swift test 6/6 green;
xcodebuild -target VoiceCatiOS -sdk iphonesimulator BUILD SUCCEEDED.

clients/apple/iOS/VoiceCatiOS/AudioSessionManager.swift

@@ -8,14 +8,12 @@ final class AudioSessionManager {
     weak var client: VoiceCatClient?
 
     func configure() {
-        let session = AVAudioSession.sharedInstance()
-        do {
-            try session.setCategory(.playAndRecord, mode: .voiceChat,
-                                    options: [.allowBluetooth, .allowBluetoothA2DP,
-                                              .defaultToSpeaker, .mixWithOthers])
-        } catch {
-            print("[AudioSession] setCategory failed: \(error)")
-        }
+        // Load stored audio routing preferences and apply them before any audio session
+        // activation. IOSAudioRouter drives all iOS audio route selection via AVAudioSession;
+        // miniaudio (the core) does NOT touch AVAudioSession on iOS.
+        IOSAudioRouter.shared.loadStoredPreferences()
+        IOSAudioRouter.shared.applyConfiguration()
+        IOSAudioRouter.shared.refreshRoutes()
 
         NotificationCenter.default.addObserver(
             self, selector: #selector(handleInterruption),
@@ -26,6 +24,10 @@ final class AudioSessionManager {
     }
 
     func activateForStreaming() throws {
+        // Re-apply the routing configuration before activating, in case the user changed
+        // settings since the last apply. The core (miniaudio) will open whatever route
+        // AVAudioSession has established.
+        IOSAudioRouter.shared.applyConfiguration()
         try AVAudioSession.sharedInstance().setActive(true, options: [])
     }
 
@@ -55,6 +57,10 @@ final class AudioSessionManager {
     }
 
     @objc private func handleRouteChange(_ notification: Notification) {
+        // Refresh the router's published state so the Settings UI updates, and re-apply
+        // the stored preferences (the new route may need the preferred input re-set).
+        IOSAudioRouter.shared.refreshRoutes()
+        IOSAudioRouter.shared.applyConfiguration()
         NotificationCenter.default.post(name: .voiceCatDeviceListChanged, object: nil)
     }
 }