fix(ios): bind export cancellation to the originating download

Ignore late picker callbacks that no longer match the active download. Add an Apple-platform regression; Swift formatting, parsing and seven portable policy tests pass locally, but the new WKWebView regression requires Xcode and has not run here.

ios/Tests/BrowserModelTests.swift

@@ -13,6 +13,18 @@ final class BrowserModelTests: XCTestCase {
         XCTAssertFalse(model.web.isInspectable)
     }
 
+    @MainActor
+    func testLateExportCancellationDoesNotEraseAnotherServersDownload() {
+        let model = BrowserModel()
+        let previous = URL(fileURLWithPath: "/old-selection.txt")
+        let current = URL(fileURLWithPath: "/new-selection.txt")
+        model.download = WebDestination(url: current)
+        model.removeDownload(expected: previous)
+        XCTAssertEqual(model.download?.url, current)
+        model.removeDownload(expected: current)
+        XCTAssertNil(model.download)
+    }
+
     @MainActor
     func testServerErasureReplacesHistoryContext() async {
         let model = BrowserModel()