refactor: remove rxjs dependency

Replace rxjs with a lightweight internal Subject implementation
to reduce package size and complexity.

src/index.ts

@@ -17,3 +17,4 @@ export type {
   RelationshipUpdate,
 } from "./observable/events";
 export type { WorldSnapshot } from "./serialization";
+export type { Observable, Subscription } from "./observable/subject";

src/observable/observe.ts

@@ -1,4 +1,4 @@
-import { Subject } from "rxjs";
+import { Subject } from "./subject";
 import type { Query } from "../query";
 import type { Entity } from "../entity";
 import type {

src/observable/subject.ts

@@ -0,0 +1,50 @@
+// ── Internal Observable / Subject ─────────────────────
+/** Minimal subscription handle returned by `.subscribe()`. */
+export interface Subscription {
+  unsubscribe(): void;
+}
+
+/** Minimal observable interface — only supports single-callback subscribe. */
+export interface Observable<T> {
+  subscribe(observer: (value: T) => void): Subscription;
+}
+
+/** Lightweight multicast subject, replacing the RxJS dependency. */
+export class Subject<T> implements Observable<T> {
+  private _subs = new Set<(value: T) => void>();
+  private _done = false;
+
+  /** Push a value to all current subscribers. No-op after complete. */
+  next(value: T): void {
+    if (this._done) return;
+    for (const fn of this._subs) {
+      fn(value);
+    }
+  }
+
+  /** Register a subscriber. Returns a handle to unsubscribe. */
+  subscribe(observer: (value: T) => void): Subscription {
+    const fn = observer;
+    this._subs.add(fn);
+    return {
+      unsubscribe: () => {
+        this._subs.delete(fn);
+      },
+    };
+  }
+
+  /** Complete this subject — clears all subscribers and silences future calls. */
+  complete(): void {
+    this._done = true;
+    this._subs.clear();
+  }
+
+  /** Return a read-only Observable facade (hides `next` / `complete`). */
+  asObservable(): Observable<T> {
+    return {
+      subscribe: (observer: (value: T) => void): Subscription => {
+        return this.subscribe(observer);
+      },
+    };
+  }
+}

src/world.ts

@@ -6,7 +6,7 @@ import { SparseSet } from "./storage/sparse-set";
 import { ObservableLayer } from "./observable/observe";
 import type { QueryUpdate, RelationshipUpdate } from "./observable/events";
 import type { RelationshipDef } from "./relationship";
-import { Observable } from "rxjs";
+import type { Observable } from "./observable/subject";
 import type { WorldEvent } from "./observable/events";
 import type { WorldSnapshot } from "./serialization";