Plugin iframes cannot trigger downloads: WKWebView cancels
shouldPerformDownload navigations when no host download handler is
registered, so blob-anchor downloads were silent no-ops on desktop.
The bridge now accepts a transferred ArrayBuffer (512 MiB cap, base64
fallback) and persists it through the native save dialog
(plugin-dialog + plugin-fs on Tauri, top-level anchor on web hosts),
resolving null when the user cancels.