Rooms, items, and endings now come from .md files under world/{rooms,items,endings}/.
Encounters still come from encounters.ts (Tasks 11–12 will complete that leg).
Cross-reference validation at module init ensures exits, item refs, and encounter
refs are all consistent. Deletes rooms.ts, items.ts, roundtrip.test.ts, and the
one-shot migration script (whose output is already committed).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
ejlewis