/**
 * Appends the server-computed `?umb__rnd=<cacheBuster>` to a clean `/App_Plugins` URL. URLs outside `/App_Plugins`,
 * URLs that already carry a query string, and an empty `cacheBuster` are returned unchanged.
 * @param {string} url The package asset URL (its own clean `/App_Plugins` path).
 * @param {string | null | undefined} cacheBuster The server-computed cache-bust value, or empty to skip stamping.
 * @returns {string} The stamped URL, or the original URL when no stamping applies.
 */
export declare function appendCacheBust(url: string, cacheBuster: string | null | undefined): string;