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/FilePreviewRepositoryTests.swift

@@ -0,0 +1,43 @@
+import XCTest
+@testable import VniDrop
+
+/// Ports `feature/send/FilePreviewRepositoryTest.kt` — persisted thumbnails,
+/// restore pruned to live transfer ids, and removal.
+@MainActor
+final class FilePreviewRepositoryTests: XCTestCase {
+
+	/// Minimal bytes that pass the PNG magic-byte check.
+	private let png = Data([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A])
+
+	private func makeRepo() -> FilePreviewRepository {
+		FilePreviewRepository(appDataDir: NSTemporaryDirectory() + "previews-" + UUID().uuidString)
+	}
+
+	func testSaveStoresPreview() {
+		let repo = makeRepo()
+		repo.save(transferId: 1, bytes: png)
+		XCTAssertEqual(repo.previews[1], png)
+	}
+
+	func testSaveRejectsNonImageBytes() {
+		let repo = makeRepo()
+		repo.save(transferId: 1, bytes: Data("not an image".utf8))
+		XCTAssertNil(repo.previews[1])
+	}
+
+	func testRestorePrunesToActiveIds() {
+		let repo = makeRepo()
+		repo.save(transferId: 1, bytes: png)
+		repo.save(transferId: 2, bytes: png)
+		repo.restore(activeTransferIds: [1])
+		XCTAssertEqual(repo.previews[1], png)
+		XCTAssertNil(repo.previews[2])
+	}
+
+	func testRemoveDeletesPreview() {
+		let repo = makeRepo()
+		repo.save(transferId: 1, bytes: png)
+		repo.remove(transferId: 1)
+		XCTAssertNil(repo.previews[1])
+	}
+}