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/AudioSessionManager.swift

@@ -0,0 +1,64 @@
+import AVFoundation
+import VoiceCatCore
+
+@MainActor
+final class AudioSessionManager {
+    static let shared = AudioSessionManager()
+
+    weak var client: VoiceCatClient?
+
+    func configure() {
+        let session = AVAudioSession.sharedInstance()
+        do {
+            try session.setCategory(.playAndRecord, mode: .voiceChat,
+                                    options: [.allowBluetooth, .allowBluetoothA2DP,
+                                              .defaultToSpeaker, .mixWithOthers])
+        } catch {
+            print("[AudioSession] setCategory failed: \(error)")
+        }
+
+        NotificationCenter.default.addObserver(
+            self, selector: #selector(handleInterruption),
+            name: AVAudioSession.interruptionNotification, object: nil)
+        NotificationCenter.default.addObserver(
+            self, selector: #selector(handleRouteChange),
+            name: AVAudioSession.routeChangeNotification, object: nil)
+    }
+
+    func activateForStreaming() throws {
+        try AVAudioSession.sharedInstance().setActive(true, options: [])
+    }
+
+    func deactivateAfterStreaming() {
+        try? AVAudioSession.sharedInstance().setActive(false,
+            options: .notifyOthersOnDeactivation)
+    }
+
+    @objc private func handleInterruption(_ notification: Notification) {
+        guard let info = notification.userInfo,
+              let typeValue = info[AVAudioSessionInterruptionTypeKey] as? UInt,
+              let type = AVAudioSession.InterruptionType(rawValue: typeValue)
+        else { return }
+
+        switch type {
+        case .began:
+            client?.audioSuspend()
+        case .ended:
+            let optionsValue = info[AVAudioSessionInterruptionOptionKey] as? UInt ?? 0
+            let options = AVAudioSession.InterruptionOptions(rawValue: optionsValue)
+            if options.contains(.shouldResume) {
+                try? AVAudioSession.sharedInstance().setActive(true)
+                client?.audioResume()
+            }
+        @unknown default: break
+        }
+    }
+
+    @objc private func handleRouteChange(_ notification: Notification) {
+        NotificationCenter.default.post(name: .voiceCatDeviceListChanged, object: nil)
+    }
+}
+
+extension Notification.Name {
+    static let voiceCatDeviceListChanged = Notification.Name("cat.voice.deviceListChanged")
+}