fix: resolve ReadableStream in simulator so LangGraph runs on RN

- Add src/polyfills.ts as the first executed module; inject ReadableStream/WritableStream/TransformStream via web-streams-polyfill and ponyfill fallback
- Import polyfills at the top of App.tsx so globals are set before any LangChain/LangGraph code loads
- Add assets/images/icon.png to fix Metro “Asset not found” for icon

src/polyfills.ts

@@ -0,0 +1,15 @@
+/**
+ * Polyfills that must run before any other app code (including LangChain/LangGraph).
+ * This file is imported as the very first line in App.tsx so that ReadableStream
+ * exists before @langchain/core or @langchain/langgraph are loaded.
+ */
+import 'web-streams-polyfill';
+
+// Ensure globalThis has ReadableStream (main polyfill may not patch in RN/Metro)
+const g = typeof globalThis !== 'undefined' ? globalThis : (typeof global !== 'undefined' ? global : (typeof self !== 'undefined' ? self : {}));
+if (typeof (g as any).ReadableStream === 'undefined') {
+  const ponyfill = require('web-streams-polyfill/dist/ponyfill.js');
+  (g as any).ReadableStream = ponyfill.ReadableStream;
+  (g as any).WritableStream = ponyfill.WritableStream;
+  (g as any).TransformStream = ponyfill.TransformStream;
+}