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/include/voicecat.h

@@ -467,6 +467,13 @@ VC_API void      vc_free_account_list(vc_account_list* list);
 /* Pull the caller's own permissions (from the last AuthResult). */
 VC_API vc_result vc_get_permissions(vc_client* c, vc_permissions* out);
 
+/* AVAudioSession interruption hooks (iOS M6). Pause/resume miniaudio device I/O for
+ * AVAudioSession interruptions (phone call, Siri, etc.) and backgrounding. Call
+ * vc_audio_suspend() when an interruption begins; call vc_audio_resume() after the
+ * session is re-activated. No-op if the audio engine is not running. */
+VC_API vc_result vc_audio_suspend(vc_client* c);
+VC_API vc_result vc_audio_resume(vc_client* c);
+
 #if defined(__cplusplus)
 } /* extern "C" */
 #endif