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/Sources/ContentView.swift

@@ -155,7 +155,7 @@ struct ContentView: View {
             AuthenticationView(destination: destination.url, model: browser)
         }
         .sheet(item: $browser.download) { file in
-            ExportView(url: file.url) { browser.removeDownload() }
+            ExportView(url: file.url) { browser.removeDownload(expected: file.url) }
         }
     }