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/component.ts

@@ -0,0 +1,33 @@
+// ── Component ─────────────────────────────────────────
+/**
+ * A component definition carries both the type shape (via TypeScript inference)
+ * and a unique key used for storage lookup.
+ *
+ * @example
+ * ```ts
+ * const Position = defineComponent({ x: 0, y: 0 });
+ * type Position = typeof Position.type;
+ * ```
+ */
+export interface ComponentDef<T extends Record<string, any>> {
+  /** Unique symbol used as the storage key. */
+  readonly _key: symbol;
+  /** Default values applied when a component is first added. */
+  readonly defaults: T;
+  /** Phantom type for inference. */
+  readonly type: T;
+}
+
+/**
+ * Define a component type. The argument provides both default values and the
+ * TypeScript shape.
+ */
+export function defineComponent<T extends Record<string, any>>(
+  defaults: T
+): ComponentDef<T> {
+  return {
+    _key: Symbol(),
+    defaults: { ...defaults },
+    type: undefined as unknown as T, // phantom; never read at runtime
+  };
+}