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/Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.9
+// swift-tools-version: 6.0
 //
 // VoiceCatCore — the shared Swift core for the VoiceCat macOS (AppKit) and iOS (SwiftUI)
 // clients. It wraps libvoicecat's C ABI (core/include/voicecat.h) as imported through the
@@ -16,12 +16,15 @@ import PackageDescription
 
 let package = Package(
     name: "VoiceCatCore",
-    // macOS 14 (Sonoma) is the AppKit client's deployment target. iOS 17 is the SwiftUI client
-    // target (clients/apple/iOS/). Run `scripts/build-xcframework.sh --all` to produce all
+    // macOS 14 (Sonoma) is the AppKit client's deployment target. iOS 18 is the SwiftUI client
+    // target (clients/apple/iOS/) — 18.0 unlocks the newest AVAudioSession APIs (stereo capture,
+    // polar patterns, data sources). Run `scripts/build-xcframework.sh --all` to produce all
     // three slices: macos-arm64, ios-arm64, ios-arm64-simulator.
+    // swift-tools-version 6.0 is required for .iOS(.v18); swiftLanguageVersions .v5 keeps the
+    // Swift 5 language mode (avoids Swift 6 strict concurrency checking on pre-existing code).
     platforms: [
         .macOS(.v14),
-        .iOS(.v17),
+        .iOS(.v18),
     ],
     products: [
         .library(name: "VoiceCatCore", targets: ["VoiceCatCore"]),
@@ -56,5 +59,6 @@ let package = Package(
                 .linkedLibrary("c++"),
             ]
         ),
-    ]
+    ],
+    swiftLanguageModes: [.v5]
 )