Each room owns a ChessEngine wrapped in a GameSession; only the server
calls insert/retract/fireRules and all EntityIds are minted server-side.
GameSessionRegistry keys sessions by room code so two rooms cannot
observe or collide with each other's working-memory state.
GameSession.applyMove validates algebraic inputs, finds the matching
legal move via ChessEngine.findMove, applies it, and returns a fact-
level diff (inserted/retracted) plus the new turn and terminal state.
Terminal states are sticky: further moves after checkmate/draw return
GAME_OVER rather than silently mutating a dead session.
Exposes @paratype/chess's headless surface (ChessEngine, coord helpers,
schema types) via a new package entry point; the React app continues to
import concrete modules directly.
