/**
 * Return the first day of the quarter for a given ISO date.
 *
 * - Returns the first day (month and day set to 1) of the quarter containing the input date.
 * - Q1 returns "01-01", Q2 returns "04-01", Q3 returns "07-01", Q4 returns "10-01".
 * - Validates input using isValidDate.
 *
 * @param value ISO PlainDate string
 * @returns ISO PlainDate string for the first day of the quarter, or "" on invalid input
 *
 * @example startOfQuarterForDate("2024-03-15") // "2024-01-01"
 * @example startOfQuarterForDate("2024-06-15") // "2024-04-01"
 * @example startOfQuarterForDate("2024-09-15") // "2024-07-01"
 * @example startOfQuarterForDate("2024-12-15") // "2024-10-01"
 * @example startOfQuarterForDate("invalid") // ""
 */
export declare function startOfQuarterForDate(value: string): string;