Room codes also dodge orphaned files on disk

makeRoomCode checked only the in-memory map, which covers every live
and restored room — but a file that failed to restore stays on disk
with no map entry, and a new room taking its code would append a
second game into the orphaned file, corrupting both. The generator
now rerolls on disk presence too.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/server/src/store.ts

@@ -3,7 +3,7 @@
 // command. Because the engine is deterministic, replaying a file rebuilds
 // the exact game state — server restarts lose nothing.
 
-import { appendFileSync, mkdirSync, readdirSync, readFileSync } from "node:fs";
+import { appendFileSync, existsSync, mkdirSync, readdirSync, readFileSync } from "node:fs";
 import { join } from "node:path";
 
 export interface RoomMetaLine {
@@ -60,6 +60,11 @@ export function ensureDataDir(): void {
   mkdirSync(DATA_DIR, { recursive: true });
 }
 
+/** A room file may exist even when the room failed to restore into memory. */
+export function roomFileExists(roomId: string): boolean {
+  return existsSync(fileFor(roomId));
+}
+
 export function appendLine(roomId: string, line: RoomLine): void {
   appendFileSync(fileFor(roomId), JSON.stringify(line) + "\n", "utf8");
 }