feat(ios): ship iOS SwiftUI client (VoiceCatiOS)

Full SwiftUI app at clients/apple/iOS/VoiceCatiOS.xcodeproj:
- 24 Swift source files: AppState + SessionState (@Observable @MainActor),
  AudioSessionManager (AVAudioSession owner + interruption/route handling),
  ServerListStore/SavedServer (App Group container + Keychain sharing),
  and 14 SwiftUI views covering the full feature set
- NavigationSplitView on iPad, TabView on iPhone (horizontalSizeClass)
- Channel tree via OutlineGroup, user list with context menu admin actions
- PTT via DragGesture(minimumDistance: 0) + @GestureState
- onEvent closures hop to MainActor via Task { @MainActor in ... }
- App Group: group.cat.voice.VoiceCat (shared with future ReplayKit extension)

C ABI: add vc_audio_suspend / vc_audio_resume (AudioEngine::suspend/resume)
called by AudioSessionManager on AVAudioSession interruption events.

XCFramework: add ios-arm64 and ios-arm64-simulator slices to build-xcframework.sh;
Package.swift gains .iOS(.v17) platform; CMakePresets.json adds apple-ios /
apple-ios-sim presets with arm64-ios / arm64-ios-simulator vcpkg triplets.

Verified: xcodebuild -target VoiceCatiOS -sdk iphonesimulator26.5 BUILD SUCCEEDED.

core/src/audio/audio_engine.h

@@ -127,6 +127,12 @@ class AudioEngine {
 
     bool running() const { return running_.load(std::memory_order_acquire); }
 
+    // Pause/resume miniaudio device I/O (called from vc_audio_suspend/resume for
+    // AVAudioSession interruptions on iOS). Thread-safe via ma_device_stop/start.
+    // Returns true on success; false if a device stop/start fails.
+    bool suspend();
+    bool resume();
+
     // Enumerate input or output devices (for UI pickers / vc_list_devices). Static: works
     // before any AudioEngine instance is running (device pickers need to populate pre-connect).
     // Inits a throwaway ma_context if VOICECAT_HAS_AUDIO; returns {} otherwise. See DeviceInfo