# Specification: Fix RRULE UNTIL Object Conversion

## Overview
The conversion of RRULE objects to strings fails for nested objects like `UNTIL` dates, resulting in `UNTIL=[object Object]`. This causes the `rrule` library to fail during expansion.

## Functional Requirements
- **Robust Value Formatting:** Implement `formatRRuleValue` to handle various types of RRULE values:
    - **Date Objects:** Convert to `YYYYMMDDTHHMMSSZ`.
    - **Nested Date Objects:** (e.g., `{ date: Date }` from `ts-ics`) Extract and convert.
    - **Other Types:** Default to string conversion.
- **Mapping Preservation:** Maintain the existing `RRULE_KEY_MAP` for key translation.

## Implementation Steps
1.  **Reproduce:** Add test `expandRecurrences - object rrule with until` in `icalendar_test.ts` using a Date object for `until`. Verify it fails with `Invalid UNTIL value`.
2.  **Fix:** Update `expandRecurrences` in `icalendar.ts` to use a formatting helper for values.
3.  **Verify:** Confirm the test case passes.

## Acceptance Criteria
- [ ] Test `expandRecurrences - object rrule with until` passes.
- [ ] The generated string correctly represents the date (e.g., `UNTIL=20260219T000000Z`).