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/Views/MainView.swift

@@ -0,0 +1,72 @@
+import SwiftUI
+
+struct MainView: View {
+    @Environment(AppState.self) private var appState
+    @Environment(\.horizontalSizeClass) private var sizeClass
+
+    var body: some View {
+        if let session = appState.session {
+            if sizeClass == .regular {
+                iPadMainView(session: session)
+            } else {
+                iPhoneMainView(session: session)
+            }
+        } else {
+            ServerListView()
+        }
+    }
+}
+
+// MARK: - iPhone layout: TabView
+
+private struct iPhoneMainView: View {
+    let session: SessionState
+
+    var body: some View {
+        TabView {
+            ChannelTreeView(session: session)
+                .tabItem {
+                    Label("Channels", systemImage: "list.bullet.indent")
+                }
+            ChatView(session: session)
+                .tabItem {
+                    Label("Chat", systemImage: "message")
+                }
+            ActivityLogView(session: session)
+                .tabItem {
+                    Label("Activity", systemImage: "bell")
+                }
+            SettingsView(session: session)
+                .tabItem {
+                    Label("Settings", systemImage: "gear")
+                }
+        }
+        .overlay(alignment: .bottom) {
+            VoiceControlsView(session: session)
+                .padding(.bottom, 56) // above tab bar
+        }
+    }
+}
+
+// MARK: - iPad layout: NavigationSplitView
+
+private struct iPadMainView: View {
+    let session: SessionState
+    @State private var columnVisibility = NavigationSplitViewVisibility.all
+
+    var body: some View {
+        NavigationSplitView(columnVisibility: $columnVisibility) {
+            ChannelTreeView(session: session)
+                .navigationTitle("Channels")
+        } content: {
+            UserListView(session: session)
+                .navigationTitle("Users")
+        } detail: {
+            VStack(spacing: 0) {
+                ChatView(session: session)
+                VoiceControlsView(session: session)
+            }
+            .navigationTitle("Chat")
+        }
+    }
+}