feat(server): add authoritative game session per room (P4.5)

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.

packages/chess/package.json

@@ -2,6 +2,12 @@
   "name": "@paratype/chess",
   "version": "0.1.0",
   "type": "module",
+  "exports": {
+    ".": {
+      "types": "./src/index.ts",
+      "default": "./src/index.ts"
+    }
+  },
   "scripts": {
     "dev": "vite",
     "build": "vite build",