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/serialization.ts

@@ -0,0 +1,10 @@
+/**
+ * Plain JSON-compatible representation of a World.
+ * Returned by `world.toJSON()`, consumed by `World.fromJSON()`.
+ */
+export interface WorldSnapshot {
+  /** Entity stable ID → component map (component name → data). */
+  entities: Record<string, Record<string, unknown>>;
+  /** Relationship name → (source ID → target ID). */
+  relationships: Record<string, Record<string, string>>;
+}