Paginated Fetch-All

pages is a fake API mapping string cursor to {'items': list of integers, 'next': string cursor or None}. Starting at start, append page items in order, including duplicates. Stop at a None next cursor, a missing cursor, or before fetching a cursor a second time. Return {'items': collected values, 'visited': cursors in request order, 'error': None, 'missing_cursor', or 'cursor_cycle'}. A missing cursor is not added to visited because no page was fetched. start may be None (empty successful traversal). No HTTP requests. Examples (JSON; null means Python None): Input: {"pages": {"a": {"items": [1, 2], "next": "b"}, "b": {"items": [2, 3], "next": null}}, "start": "a"} Output: {"items": [1, 2, 2, 3], "visited": ["a", "b"], "error": null} Input: {"pages": {"a": {"items": [1], "next": "a"}}, "start": "a"} Output: {"items": [1], "visited": ["a"], "error": "cursor_cycle"}

def fetch_all(pages: dict, start: str | None) -> dict:
    """Follow fake API cursors once each and report broken chains."""
    pass