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/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.