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/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,