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/Tests/SettingsModelTests.swift

@@ -0,0 +1,45 @@
+import XCTest
+@testable import VniDrop
+
+/// Ports settings assertions from `feature/ViewModelsTest.kt` — username debounce
+/// persistence and the Storage "delete all transfers" flow.
+@MainActor
+final class SettingsModelTests: XCTestCase {
+
+	private func makeModel(_ core: FakeCoreGateway, preferences: AppPreferencesRepository) -> SettingsModel {
+		SettingsModel(
+			environment: PlatformEnvironment(name: "Test", appVersion: "0.1.0", defaultCoreDataDir: NSTemporaryDirectory()),
+			deviceInfoProvider: FakeDeviceInfoProvider(),
+			fileSystemService: FakeFileSystemService(),
+			repository: core,
+			preferences: preferences,
+			notifications: LocalNotificationService(),
+			messages: UiMessageController(),
+			bugReports: NoopBugReportService(),
+			diagnosticsIncluded: false
+		)
+	}
+
+	func testUsernameChangeDebouncesAndPersists() async {
+		let prefs = Fixtures.preferences(username: "Original")
+		let model = makeModel(FakeCoreGateway(), preferences: prefs)
+
+		model.setUsername("Alice")
+		XCTAssertEqual(model.state.username, "Alice") // immediate local echo
+		await waitUntil { prefs.preferences.username == "Alice" } // persisted after debounce
+		XCTAssertEqual(prefs.preferences.username, "Alice")
+	}
+
+	func testDeleteAllTransfersDeletesEveryTransfer() async {
+		let core = FakeCoreGateway()
+		let model = makeModel(core, preferences: Fixtures.preferences())
+		core.setState(CoreState(isInitialized: true, transfers: [
+			Fixtures.transfer(id: 2, direction: .send, status: .sharing),
+			Fixtures.transfer(id: 3, direction: .receive, status: .done),
+		]))
+
+		model.deleteAllTransfers()
+		await waitUntil { core.deletedTransfers.count == 2 }
+		XCTAssertEqual(Set(core.deletedTransfers), [2, 3])
+	}
+}