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/AppModelTests.swift

@@ -0,0 +1,41 @@
+import XCTest
+@testable import VniDrop
+
+/// Ports app-level assertions: core initialization on launch, destination
+/// selection guard, and theme following preferences.
+@MainActor
+final class AppModelTests: XCTestCase {
+
+	private func makeModel(_ core: FakeCoreGateway, preferences: AppPreferencesRepository) -> AppModel {
+		AppModel(
+			environment: PlatformEnvironment(name: "Test", appVersion: "0.1.0", defaultCoreDataDir: NSTemporaryDirectory()),
+			repository: core,
+			preferences: preferences,
+			messages: UiMessageController()
+		)
+	}
+
+	func testInitializesCoreOnLaunch() async {
+		let core = FakeCoreGateway()
+		_ = makeModel(core, preferences: Fixtures.preferences())
+		await waitUntil { core.state.isInitialized }
+		XCTAssertTrue(core.state.isInitialized)
+	}
+
+	func testSelectDestination() {
+		let model = makeModel(FakeCoreGateway(), preferences: Fixtures.preferences())
+		XCTAssertEqual(model.destination, .send)
+		model.selectDestination(.settings)
+		XCTAssertEqual(model.destination, .settings)
+		model.selectDestination(.settings) // no-op guard
+		XCTAssertEqual(model.destination, .settings)
+	}
+
+	func testThemeModeFollowsPreferences() async {
+		let prefs = Fixtures.preferences()
+		let model = makeModel(FakeCoreGateway(), preferences: prefs)
+		prefs.setThemeMode(.dark)
+		await waitUntil { model.themeMode == .dark }
+		XCTAssertEqual(model.themeMode, .dark)
+	}
+}