test(apple): CoreGateway seam, XCTest suite, and CI

- Introduce a CoreGateway protocol so feature models depend on a seam
  (CoreRepository conforms); enables faking the core in tests
- Add a VniDropTests target with 42 tests mirroring the KMP suites:
  approval coordinator, send/receive/settings/app models, preferences,
  file previews, invitation decode, message queue, error mapping
- Add a fake gateway/file-system/device-info and fixtures
- Add .github/workflows/apple.yml: build the Rust core, generate the
  project, and run the tests on an iOS Simulator

apple/VniDrop/Features/App/AppModel.swift

@@ -9,13 +9,13 @@ final class AppModel: ObservableObject {
 	@Published private(set) var themeMode: ThemeMode = .system
 
 	private let environment: PlatformEnvironment
-	private let repository: CoreRepository
+	private let repository: CoreGateway
 	private let messages: UiMessageController
 	private var cancellables = Set<AnyCancellable>()
 
 	init(
 		environment: PlatformEnvironment,
-		repository: CoreRepository,
+		repository: CoreGateway,
 		preferences: AppPreferencesRepository,
 		messages: UiMessageController
 	) {