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/iOS/VoiceCatiOS/BroadcastCredentials.swift

@@ -0,0 +1,21 @@
+import Foundation
+
+struct BroadcastCredentials: Codable {
+    var host: String
+    var port: Int
+    var authMode: String
+    var username: String
+    var tofuPinsPath: String
+
+    static func loadFromAppGroup() -> BroadcastCredentials? {
+        let groupId = "group.cat.voice.VoiceCat"
+        guard let container = FileManager.default.containerURL(
+            forSecurityApplicationGroupIdentifier: groupId)
+        else { return nil }
+        let url = container
+            .appendingPathComponent("voicecat", isDirectory: true)
+            .appendingPathComponent("broadcast_credentials.json")
+        guard let data = try? Data(contentsOf: url) else { return nil }
+        return try? JSONDecoder().decode(BroadcastCredentials.self, from: data)
+    }
+}