feat: add support for data-carrying relationships

Introduce the ability to attach optional data payloads to
relationships. This includes:

- Updating `defineRelationship` to accept default values.
- Adding `getRelData` and `setRelData` to the `World` class.
- Allowing `relate` to accept an optional data override.
- Updating serialization to include relationship data in snapshots.
- Implementing lazy storage for relationship data using `SparseSet`.

src/serialization.ts

@@ -5,6 +5,9 @@
 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>>;
+  /** Relationship name → (source ID → target ID or edge object). */
+  relationships: Record<
+    string,
+    Record<string, string | { target: string; data: unknown }>
+  >;
 }