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.

clients/apple/Package.swift

@@ -16,11 +16,12 @@ import PackageDescription
 
 let package = Package(
     name: "VoiceCatCore",
-    // macOS 14 (Sonoma) is the AppKit client's deployment target. iOS is deferred; when the
-    // iOS app lands, add `.iOS(.v17)` here and the apple-ios / apple-ios-sim slices to the
-    // XCFramework (scripts/build-xcframework.sh --all).
+    // 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
+    // three slices: macos-arm64, ios-arm64, ios-arm64-simulator.
     platforms: [
         .macOS(.v14),
+        .iOS(.v17),
     ],
     products: [
         .library(name: "VoiceCatCore", targets: ["VoiceCatCore"]),