feat(smartstate): Add middleware, computed, batching, selector memoization, AbortSignal support, and Web Component Context Protocol provider

ts/smartstate.classes.computed.ts

@@ -0,0 +1,16 @@
+import * as plugins from './smartstate.plugins.js';
+import { combineLatest, map } from 'rxjs';
+import type { StatePart } from './smartstate.classes.statepart.js';
+
+/**
+ * creates a computed observable derived from multiple state parts.
+ * the observable is lazy — it only subscribes to sources when subscribed to.
+ */
+export function computed<TResult>(
+  sources: StatePart<any, any>[],
+  computeFn: (...states: any[]) => TResult,
+): plugins.smartrx.rxjs.Observable<TResult> {
+  return combineLatest(sources.map((sp) => sp.select())).pipe(
+    map((states) => computeFn(...states)),
+  ) as plugins.smartrx.rxjs.Observable<TResult>;
+}