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/Views/VoiceControlsView.swift

@@ -6,7 +6,7 @@ struct VoiceControlsView: View {
 
     var body: some View {
         HStack(spacing: 20) {
-            // Mic toggle / PTT button
+            // Join/Leave Voice button (mirrors macOS micToggleButton)
             if session.voiceState.inputMode == .pushToTalk {
                 PTTButton(session: session)
             } else {
@@ -17,14 +17,16 @@ struct VoiceControlsView: View {
                         session.startMicStream()
                     }
                 } label: {
-                    Image(systemName: session.voiceState.micActive ? "mic.fill" : "mic.slash.fill")
-                        .font(.title2)
-                        .frame(width: 44, height: 44)
-                        .background(session.voiceState.micActive ? Color.green : Color(.systemGray4), in: Circle())
-                        .foregroundStyle(session.voiceState.micActive ? .white : .primary)
+                    Text(session.voiceState.micActive ? "Leave Voice" : "Join Voice")
+                        .font(.body.weight(.semibold))
+                        .frame(minWidth: 110)
+                        .padding(.vertical, 8)
+                        .padding(.horizontal, 12)
+                        .background(session.voiceState.micActive ? Color.green.opacity(0.2) : Color.accentColor.opacity(0.15), in: RoundedRectangle(cornerRadius: 8))
+                        .foregroundStyle(session.voiceState.micActive ? .green : .accentColor)
                 }
                 .disabled(session.currentChannelId == 0)
-                .accessibilityLabel(session.voiceState.micActive ? "Stop microphone" : "Start microphone")
+                .accessibilityLabel(session.voiceState.micActive ? "Leave Voice — stop sending microphone audio" : "Join Voice — start sending microphone audio")
             }
 
             // Level meter
@@ -34,7 +36,7 @@ struct VoiceControlsView: View {
 
             Spacer()
 
-            // Self mute
+            // Self mute (disabled when not in voice)
             Button {
                 session.setMute(!session.voiceState.selfMuted, deafened: session.voiceState.selfDeafened)
             } label: {
@@ -42,9 +44,10 @@ struct VoiceControlsView: View {
                     .font(.title3)
                     .foregroundStyle(session.voiceState.selfMuted ? .red : .primary)
             }
+            .disabled(!session.voiceState.micActive)
             .accessibilityLabel(session.voiceState.selfMuted ? "Unmute microphone" : "Mute microphone")
 
-            // Self deafen
+            // Self deafen (disabled when not in voice)
             Button {
                 session.setMute(session.voiceState.selfMuted, deafened: !session.voiceState.selfDeafened)
             } label: {
@@ -52,6 +55,7 @@ struct VoiceControlsView: View {
                     .font(.title3)
                     .foregroundStyle(session.voiceState.selfDeafened ? .red : .primary)
             }
+            .disabled(!session.voiceState.micActive)
             .accessibilityLabel(session.voiceState.selfDeafened ? "Undeafen" : "Deafen")
 
             // Disconnect