feat: implement relationship system

src/relationship.ts

@@ -0,0 +1,22 @@
+// ── Relationship ─────────────────────────────────────
+/**
+ * A relationship definition — like a component, but represents a directed
+ * link between two entities.
+ *
+ * @example
+ * ```ts
+ * const ChildOf = defineRelationship();
+ * world.relate(child, ChildOf, parent);
+ * ```
+ */
+export interface RelationshipDef {
+  /** Unique symbol used as the storage key. */
+  readonly _key: symbol;
+}
+
+/**
+ * Define a named relationship between entities.
+ */
+export function defineRelationship(): RelationshipDef {
+  return { _key: Symbol() };
+}