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/UiFeedbackTests.swift

@@ -0,0 +1,53 @@
+import XCTest
+@testable import VniDrop
+
+/// Ports `ui/feedback/UiMessageControllerTest.kt` and `UserFacingErrorTest.kt`.
+@MainActor
+final class UiMessageControllerTests: XCTestCase {
+
+	func testQueuesAndAdvances() {
+		let c = UiMessageController()
+		c.show(UiMessage(text: .dynamic("first")))
+		c.show(UiMessage(text: .dynamic("second")))
+		XCTAssertEqual(c.current?.text, .dynamic("first"))
+
+		c.advance()
+		XCTAssertEqual(c.current?.text, .dynamic("second"))
+
+		c.advance()
+		XCTAssertNil(c.current)
+	}
+
+	func testErrorSuppressesUserCancellation() {
+		let c = UiMessageController()
+		c.error(InvitationError.message("QR scanning was cancelled"))
+		XCTAssertNil(c.current) // cancellations are swallowed
+	}
+
+	func testErrorShowsNonCancellation() {
+		let c = UiMessageController()
+		c.error(InvitationError.message("The transfer was refused"))
+		XCTAssertEqual(c.current?.tone, .error)
+	}
+}
+
+@MainActor
+final class UserFacingErrorTests: XCTestCase {
+
+	func testIsUserCancellation() {
+		XCTAssertTrue(InvitationError.message("NFC reading was cancelled").isUserCancellation)
+		XCTAssertTrue(InvitationError.message("User canceled the picker").isUserCancellation)
+		XCTAssertFalse(InvitationError.message("A database error occurred").isUserCancellation)
+	}
+
+	func testToUiTextMapsKnownReasons() {
+		XCTAssertEqual(InvitationError.message("The transfer was refused").toUiText(), .resource("error_permission"))
+		XCTAssertEqual(InvitationError.message("invalid ticket").toUiText(), .resource("error_invalid_ticket"))
+		XCTAssertEqual(InvitationError.message("Select at least one file to share").toUiText(), .resource("error_share_empty"))
+		XCTAssertEqual(InvitationError.message("Camera access is required").toUiText(), .resource("error_camera"))
+	}
+
+	func testToUiTextFallsBackToGeneric() {
+		XCTAssertEqual(InvitationError.message("something entirely unexpected").toUiText(), .resource("error_generic"))
+	}
+}