# Specification: Generic RRULE Object Formatter

## Overview
The current implementation for converting RRULE objects (from `ts-ics`) back to strings is brittle and fails when encountering nested objects or arrays for properties like `BYDAY`. This results in `[object Object]` being injected into the RRULE string, causing syntax errors in the `rrule` library. This track replaces the specific property handling with a robust, recursive generic formatter.

## Functional Requirements
- **Recursive Value Formatting:** The `formatRRuleValue` function must handle nested structures:
    - **Arrays:** Join elements with commas (e.g., `['MO', 'TU']` -> `MO,TU`).
    - **Date Objects:** Format as `YYYYMMDDTHHMMSSZ`.
    - **Nested Property Objects:** If an object has a `date`, `day`, or `value` property, extract and format that value (e.g., `{ day: 'MO' }` -> `MO`).
    - **Recursion:** If an array contains objects, or an object contains an array, the logic must recurse until primitives are reached.
- iCalendar Compliance: Ensure the resulting string for every property matches the iCalendar spec format expected by `rrule.js`.

## Implementation Steps
1. Reproduce: Create a test case in `icalendar_test.ts` where `BYDAY` is an object or an array of objects. Verify it fails with `Invalid weekday string: [object Object]`.
2. Fix: Re-implement `formatRRuleValue` in `icalendar.ts` as a recursive function that handles Arrays, Dates, and standard `ts-ics` nested value objects.
3. Verify: 
    - Confirm the `BYDAY` reproduction test passes.
    - Add a composite test with `UNTIL` (Date) and `BYDAY` (Array) together.
    - Run regression tests to ensure standard string RRULEs still work.

## Acceptance Criteria
- [ ] `expandRecurrences` correctly handles `BYDAY` when provided as an array of objects.
- [ ] `expandRecurrences` correctly handles `UNTIL` when provided as a nested date object.
- [ ] All 12+ existing tests pass, including regression checks for string-based RRULEs.