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.

CMakeLists.txt

@@ -5,6 +5,13 @@ project(voicecat
     DESCRIPTION "Self-hosted native voice & text chat (see docs/)"
     LANGUAGES CXX)
 
+# On iOS, audio_engine.cpp includes miniaudio.h which pulls in AVFoundation Objective-C
+# headers. Those cannot be compiled as C++; we set audio_engine.cpp's LANGUAGE to OBJCXX
+# in core/CMakeLists.txt, but that requires the language to be enabled first.
+if(CMAKE_SYSTEM_NAME STREQUAL "iOS")
+    enable_language(OBJCXX)
+endif()
+
 # ── Options ───────────────────────────────────────────────────────────────────
 # The M0 skeleton compiles with NO third-party dependencies: every subsystem is a
 # stub that returns VC_ERR_NOT_IMPLEMENTED. As each subsystem is built out, flip