fix(decorators): Add Symbol.metadata polyfill and import it at entry to ensure decorator metadata is available

ts/shim.ts

@@ -0,0 +1,6 @@
+/**
+ * Polyfill for Symbol.metadata required by TC39 Stage 3 decorators.
+ * Must be imported before any decorator code loads.
+ * @see https://github.com/tc39/proposal-decorator-metadata
+ */
+(Symbol as any).metadata ??= Symbol.for('Symbol.metadata');