feat: implement core ECS engine with RxJS observability

Initial implementation of an Entity-Component-System (ECS) featuring:
- Sparse set-based component storage for efficient access.
- Entity lifecycle management with generation-based recycling.
- Reactive query system using RxJS for change tracking.
- Batched event flushing to support frame-based updates.
- Type-safe component definitions via TypeScript inference.

src/index.ts

@@ -0,0 +1,10 @@
+// ── 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';