/**
 * A path in `folder` that nothing occupies, suffixed the way Explorer does.
 *
 * Checked with `fs` rather than remembered: the folder belongs to the user,
 * and anything may have appeared in it between the choice and this call.
 * Shared by every route that writes into a folder of the user's choosing —
 * the drop delivery and the context menu's directory download — so the two
 * cannot drift apart on how a collision is avoided.
 */
export declare function freeLocalName(folder: string, name: string): string;