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.

cmake/toolchains/ios-device.cmake

@@ -0,0 +1,8 @@
+# Chainload toolchain for iOS device cross-compilation (arm64-ios vcpkg triplet).
+# Loaded by vcpkg when building all dependencies for the arm64-ios target, ensuring
+# every dep is compiled against the iPhone OS SDK (not the macOS SDK).
+set(CMAKE_SYSTEM_NAME iOS)
+set(CMAKE_SYSTEM_PROCESSOR arm64)
+set(CMAKE_OSX_ARCHITECTURES arm64)
+set(CMAKE_OSX_SYSROOT iphoneos)
+set(CMAKE_OSX_DEPLOYMENT_TARGET 17.0)