fix(server): allow define() to be called before start() (#370)

.changeset/fix-room-define-timing.md

@@ -0,0 +1,13 @@
+---
+"@esengine/server": patch
+---
+
+fix: allow define() to be called before start()
+
+Previously, calling `server.define()` before `server.start()` would throw an error because `roomManager` was initialized inside `start()`. This fix moves the `roomManager` initialization to `createServer()`, allowing the expected usage pattern:
+
+```typescript
+const server = await createServer({ port: 3000 })
+server.define('world', WorldRoom)  // Now works correctly
+await server.start()
+```