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/InvitationTests.swift

@@ -0,0 +1,43 @@
+import XCTest
+@testable import VniDrop
+
+/// Ports `feature/receive/ExternalInvitationControllerTest.kt` + the `.vnd`
+/// decode/filename helpers.
+@MainActor
+final class InvitationTests: XCTestCase {
+
+	func testValidateInvitationAcceptsValid() {
+		guard case .success(let raw) = validateInvitation("some-ticket") else { return XCTFail("expected success") }
+		XCTAssertEqual(raw, "some-ticket")
+	}
+
+	func testValidateRejectsEmpty() {
+		guard case .failure(let error) = validateInvitation("   \n ") else { return XCTFail("expected failure") }
+		XCTAssertTrue((error as? InvitationError) != nil)
+	}
+
+	func testValidateRejectsTooLarge() {
+		let big = String(repeating: "a", count: maxVniDropInvitationBytes + 1)
+		guard case .failure = validateInvitation(big) else { return XCTFail("expected failure") }
+	}
+
+	func testDecodeInvitationBytesRoundTrip() throws {
+		let text = "vnidrop://ticket-abc"
+		let decoded = try decodeInvitationBytes(Data(text.utf8))
+		XCTAssertEqual(decoded, text)
+	}
+
+	func testDecodeRejectsEmptyData() {
+		XCTAssertThrowsError(try decodeInvitationBytes(Data()))
+	}
+
+	func testDecodeRejectsInvalidUtf8() {
+		XCTAssertThrowsError(try decodeInvitationBytes(Data([0xFF, 0xFE, 0xFD])))
+	}
+
+	func testInvitationFileNameSanitizes() {
+		XCTAssertEqual(invitationFileName("My Photos"), "My-Photos.vnd")
+		XCTAssertEqual(invitationFileName("  "), "invitation.vnd")
+		XCTAssertTrue(invitationFileName("a/b:c*d").hasSuffix(".vnd"))
+	}
+}