/**
 * Calculate the week number based on configurable week start day.
 *
 * - ISO (monday): Week 1 is the week containing the first Thursday (has at least 4 days in the year)
 * - US (sunday): Week 1 is the week containing Jan 1 (first week has at least 1 day)
 *
 * @param dateStr ISO PlainDate string (e.g. "2024-03-15")
 * @param weekStartsOn optional: "monday" (default) or "sunday"
 * @returns Week number (1-53), or null on invalid input
 *
 * @example getWeekNumber("2024-01-01") // 1
 * @example getWeekNumber("2024-01-08") // 2
 * @example getWeekNumber("2024-12-31") // 1
 * @example getWeekNumber("2024-01-01", "sunday") // 1
 * @example getWeekNumber("invalid") // null
 */
export declare function getWeekNumber(dateStr: string, weekStartsOn?: "monday" | "sunday"): number | null;