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/Sources/VoiceCatCore/VoiceCatClient.swift

@@ -301,6 +301,14 @@ public final class VoiceCatClient {
         VoiceCatResult(vc_set_input_device(handle, streamId, deviceId))
     }
 
+    /// Set the capture channel count for a local MIC stream (1 = mono, 2 = stereo interleaved).
+    /// Takes effect on the next AudioEngine restart (immediately if already running). Used by
+    /// the iOS `IOSAudioRouter` when the user picks stereo built-in mic capture.
+    @discardableResult
+    public func setCaptureChannels(streamId: UInt32, channels: UInt32) -> VoiceCatResult {
+        VoiceCatResult(vc_set_capture_channels(handle, streamId, channels))
+    }
+
     @discardableResult
     public func setInputMode(_ mode: VoiceCatInputMode) -> VoiceCatResult {
         VoiceCatResult(vc_set_input_mode(handle, mode.cValue))