/**
 * Return a specific time unit extracted from a PlainTime string.
 *
 * - Extracts "hour", "minute", "second", or "millisecond" from a PlainTime.
 * - Returns zero-padded string for hour, minute, second (2 digits) and millisecond (3 digits).
 * - Returns "" for invalid input.
 *
 * @param value ISO PlainTime string
 * @param unit unit to extract from the time
 * @returns string representation of the requested unit or "" on invalid input
 *
 * @example parseUnitFromTime("14:30:45.123", "hour") // "14"
 * @example parseUnitFromTime("14:30:45.123", "minute") // "30"
 * @example parseUnitFromTime("14:30:45.123", "second") // "45"
 * @example parseUnitFromTime("14:30:45.123", "millisecond") // "123"
 * @example parseUnitFromTime("invalid", "hour") // ""
 */
export declare function parseUnitFromTime(value: string, unit: "hour" | "minute" | "second" | "millisecond"): string;