/**
 * Return true if `start` and `end` form a valid time interval — both parseable as
 * ISO PlainTime strings and `start <= end`.
 *
 * - Both inputs must be ISO 8601 time strings (e.g. `"14:30:00"`).
 * - Equal `start === end` is valid.
 * - Invalid input, malformed strings, or leap-second strings return `false`.
 *
 * @param start ISO 8601 time string (interval start)
 * @param end ISO 8601 time string (interval end)
 * @returns true if start and end form a valid time interval, or false on invalid input
 *
 * @example isValidTimeInterval("09:00:00", "17:00:00") // true
 * @example isValidTimeInterval("12:00:00", "12:00:00") // true
 * @example isValidTimeInterval("17:00:00", "09:00:00") // false
 * @example isValidTimeInterval("invalid", "12:00:00") // false
 */
export declare function isValidTimeInterval(start: string, end: string): boolean;