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/Core/CoreGateway.swift

@@ -0,0 +1,38 @@
+import Foundation
+import Combine
+import VnidropCore
+
+/// Seam between the feature models and the Rust core, mirroring `CoreGateway`
+/// in the KMP `shared` module. `CoreRepository` is the production implementation;
+/// tests substitute a fake so the models can be exercised without the FFI.
+@MainActor
+protocol CoreGateway: AnyObject {
+	/// Latest published core state.
+	var state: CoreState { get }
+	/// Publisher of core-state changes (the models subscribe to this).
+	var statePublisher: AnyPublisher<CoreState, Never> { get }
+	/// Coalesced change hints emitted by the event sink.
+	var signals: AnyPublisher<CoreSignal, Never> { get }
+
+	func initialize(appDataDir: String) async -> Result<Void, Error>
+	func shutdown()
+	func shareSources(
+		_ sources: [ShareSource],
+		transferName: String,
+		senderName: String,
+		accessPolicy: ShareAccessPolicy
+	) async -> Result<Share, Error>
+	func inspectTicket(_ ticket: String) async -> Result<TicketInspectionModel, Error>
+	func receive(ticket: String, outputDir: String, receiverName: String) async -> Result<Void, Error>
+	func receiveIntoSecurityScopedDirectory(
+		ticket: String,
+		outputDirectoryUrl: String,
+		receiverName: String
+	) async -> Result<Void, Error>
+	func cancel(transferId: UInt64) async -> Result<Void, Error>
+	func delete(transferId: UInt64) async -> Result<Void, Error>
+	func clearReceiveHistory() async -> Result<UInt64, Error>
+	func receiverRequests(transferId: UInt64) async -> Result<[ReceiverRequestModel], Error>
+	func respondReceiverRequest(requestId: String, accepted: Bool, reason: String?) async -> Result<Void, Error>
+	func refresh() async -> Result<Void, Error>
+}

apple/VniDrop/Core/CoreRepository.swift

@@ -9,8 +9,9 @@ import Combine
 /// UniFFI calls block (the core drives its own runtime via `block_on`), so they
 /// run on a background queue and results are hopped back to the main actor.
 @MainActor
-final class CoreRepository: ObservableObject {
+final class CoreRepository: ObservableObject, CoreGateway {
 	@Published private(set) var state = CoreState()
+	var statePublisher: AnyPublisher<CoreState, Never> { $state.eraseToAnyPublisher() }
 
 	private let signalsSubject = PassthroughSubject<CoreSignal, Never>()
 	/// Coalesced change hints; subscribe to react to approval/history/transfer changes.

apple/VniDrop/Core/FileSystemService.swift

@@ -30,7 +30,7 @@ protocol FileSystemService {
 	/// Releases only app-owned picker copies; never deletes original user sources.
 	func discardPickedFiles(_ files: [PickedShareFile]) async
 	func sharePickedFiles(
-		repository: CoreRepository,
+		repository: CoreGateway,
 		files: [PickedShareFile],
 		transferName: String,
 		senderName: String,