feat: add world serialization support

Introduce `toJSON` and `fromJSON` methods to the `World` class to
allow saving and restoring world states. This requires components and
relationships to have human-readable names for stable serialization.

src/component.ts

@@ -5,13 +5,15 @@
  *
  * @example
  * ```ts
- * const Position = defineComponent({ x: 0, y: 0 });
+ * const Position = defineComponent('position', { x: 0, y: 0 });
  * type Position = typeof Position.type;
  * ```
  */
 export interface ComponentDef<T extends Record<string, any>> {
   /** Unique symbol used as the storage key. */
   readonly _key: symbol;
+  /** Human-readable name, used for serialization. */
+  readonly name: string;
   /** Default values applied when a component is first added. */
   readonly defaults: T;
   /** Phantom type for inference. */
@@ -19,15 +21,17 @@ export interface ComponentDef<T extends Record<string, any>> {
 }
 
 /**
- * Define a component type. The argument provides both default values and the
- * TypeScript shape.
+ * Define a component type. The name is used for serialization.
+ * The defaults object provides both the TypeScript shape and initial values.
  */
 export function defineComponent<T extends Record<string, any>>(
-  defaults: T
+  name: string,
+  defaults: T,
 ): ComponentDef<T> {
   return {
     _key: Symbol(),
+    name,
     defaults: { ...defaults },
-    type: undefined as unknown as T, // phantom; never read at runtime
+    type: undefined as unknown as T,
   };
 }