/**
 * Get the ISO week number of the current date.
 *
 * - Uses Temporal.Now.plainDateTimeISO to get current date in system timezone.
 * - Uses Temporal.PlainDate.weekOfYear for ISO week number.
 * - Returns null when system timezone is unavailable.
 *
 * @returns The ISO week number (1-53), or null on invalid
 *
 * @example getWeekOfYear() // 15
 * @example getWeekOfYear() // null (when system timeZone unavailable)
 */
export declare function getWeekOfYear(): number | null;