feat: add relationship support and observability

Introduces relationship management to the World, including
storage for forward and reverse relationships. Adds new observable
events and a `RelationshipUpdate` stream to track when relationships
are added or removed.

src/index.ts

@@ -1,10 +1,16 @@
 // ── Public API ─────────────────────────────────────────
-export { World } from './world';
-export { defineComponent } from './component';
-export type { ComponentDef } from './component';
-export { query } from './query';
-export { Query } from './query';
-export type { Entity } from './entity';
-export { makeEntity, entityIndex, entityGeneration } from './entity';
-export { SparseSet } from './storage/sparse-set';
-export type { WorldEvent, QueryUpdate } from './observable/events';
+export { World } from "./world";
+export { defineComponent } from "./component";
+export type { ComponentDef } from "./component";
+export { defineRelationship } from "./relationship";
+export type { RelationshipDef } from "./relationship";
+export { query } from "./query";
+export { Query } from "./query";
+export type { Entity } from "./entity";
+export { makeEntity, entityIndex, entityGeneration } from "./entity";
+export { SparseSet } from "./storage/sparse-set";
+export type {
+  WorldEvent,
+  QueryUpdate,
+  RelationshipUpdate,
+} from "./observable/events";