Recurring-event expander

Implement expand(rule: Rule, rangeStart: string, rangeEnd: string): string[] in recurrence.ts. Dates are ISO YYYY-MM-DD strings; treat everything as calendar dates with no time zones. Rule = {start: string; freq: "daily"|"weekly"|"monthly"; interval?: number; byWeekday?: ("MO"|"TU"|"WE"|"TH"|"FR"|"SA"|"SU")[]; byMonthDay?: number; count?: number; until?: string; exceptions?: string[]}. Semantics: interval defaults to 1. weekly with byWeekday yields those weekdays in each qualifying week (weeks start Monday). monthly with byMonthDay yields that day; if a month lacks it (e.g. 31), skip that month. byMonthDay: -1 means last day of month. count caps total occurrences from start, counted before range filtering. until is inclusive. exceptions are removed but still count toward count. Return occurrences within [rangeStart, rangeEnd] inclusive, sorted. Export the Rule type. No dependencies. Return only the file in one code block.