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/Approvals/ApprovalCoordinator.swift

@@ -26,7 +26,7 @@ struct ApprovalState: Equatable {
 final class ApprovalCoordinator: ObservableObject {
 	@Published private(set) var state = ApprovalState()
 
-	private let repository: CoreRepository
+	private let repository: CoreGateway
 	private let preferences: AppPreferencesRepository
 	private let notifications: LocalNotificationService
 	private let visibility: AppVisibility
@@ -36,7 +36,7 @@ final class ApprovalCoordinator: ObservableObject {
 	private var cancellables = Set<AnyCancellable>()
 
 	init(
-		repository: CoreRepository,
+		repository: CoreGateway,
 		preferences: AppPreferencesRepository,
 		notifications: LocalNotificationService,
 		visibility: AppVisibility,
@@ -57,7 +57,7 @@ final class ApprovalCoordinator: ObservableObject {
 			}
 			.store(in: &cancellables)
 
-		repository.$state
+		repository.statePublisher
 			.sink { [weak self] core in
 				guard let self, core.isInitialized else { return }
 				let sharing = core.transfers.filter { $0.direction == .send && $0.status == .sharing }