/**
 * Ensures that a given URL or path string starts with a leading slash (`/`).
 *
 * - If the string already starts with a slash, it is returned unchanged.
 * - If it does not, a leading slash is prepended.
 *
 * @param url - The URL or path to normalize.
 * @returns The same string, guaranteed to start with `/`.
 *
 * @example
 * addLeadingSlash('about');   // "/about"
 * addLeadingSlash('/about');  // "/about"
 * addLeadingSlash('');        // "/"
 */
export declare function addLeadingSlash(url: string): string;
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