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/Models.swift

@@ -135,6 +135,52 @@ public struct Device: Sendable, Equatable, Identifiable {
     }
 }
 
+/// iOS audio input port — derived from `AVAudioSession.availableInputs`. Unlike the
+/// miniaudio-based `Device` (which returns ~2 entries on iOS), this exposes the real
+/// AVAudioSession input ports (builtInMic, bluetoothHFP, headsetMic, usbAudio, airPlay)
+/// with their data sources (orientation: front/back/top/bottom) and polar patterns
+/// (omni/cardioid/subcardioid/bidirectional). Used by `IOSAudioRouter` + `SettingsView`.
+public struct IOSAudioInputPort: Identifiable, Hashable {
+    public let id: String           // port UID (stable across route changes)
+    public let name: String         // human-readable port name
+    public let portType: String     // AVAudioSession.Port raw value as string
+    public let dataSources: [IOSAudioDataSource]?
+    public let isSelected: Bool     // true if this is the current preferredInput
+
+    public init(id: String, name: String, portType: String,
+                dataSources: [IOSAudioDataSource]?, isSelected: Bool) {
+        self.id = id; self.name = name; self.portType = portType
+        self.dataSources = dataSources; self.isSelected = isSelected
+    }
+}
+
+/// iOS audio data source — a sub-selection of an input port (e.g. built-in mic
+/// orientation: front/back/top/bottom). May have polar pattern options.
+public struct IOSAudioDataSource: Identifiable, Hashable {
+    public let id: String               // dataSource UID
+    public let name: String             // "Front", "Back", "Top", "Bottom"
+    public let polarPatterns: [String]? // AVAudioSession.PolarPattern raw values
+    public let isSelected: Bool         // true if this is the current preferredDataSource
+    public let selectedPolarPattern: String?
+
+    public init(id: String, name: String, polarPatterns: [String]?,
+                isSelected: Bool, selectedPolarPattern: String?) {
+        self.id = id; self.name = name; self.polarPatterns = polarPatterns
+        self.isSelected = isSelected; self.selectedPolarPattern = selectedPolarPattern
+    }
+}
+
+/// iOS audio output route — read-only display of `AVAudioSession.currentRoute.outputs`.
+public struct IOSAudioOutputRoute: Identifiable, Hashable {
+    public let id: String       // port UID
+    public let name: String     // human-readable route name
+    public let portType: String // AVAudioSession.Port raw value as string
+
+    public init(id: String, name: String, portType: String) {
+        self.id = id; self.name = name; self.portType = portType
+    }
+}
+
 /// Effective Opus configuration — mirrors `vc_audio_config`.
 public struct AudioConfig: Sendable, Equatable {
     public let codec: UInt32            // 0 = OPUS