feat(core-shared/instrumentation): withSpan attaches runtime __instrumented marker

packages/core-shared/src/instrumentation/with-span.ts

@@ -1,5 +1,6 @@
 import type { ITracer, SpanOpts } from "./tracer.interface";
 import type { Instrumented } from "../conformance/brands";
+import { attachBrand } from "../conformance/brand-runtime";
 
 export function withSpan<Args extends unknown[], R, Extra extends object>(
   tracer: ITracer,
@@ -20,7 +21,8 @@ export function withSpan<Args extends unknown[], R>(
     const resolved = typeof opts === "function" ? opts(args) : opts;
     return tracer.startSpan(resolved, () => fn(...args));
   };
-  // Cast is the only runtime concession — the brand is a phantom type;
-  // there is no real `__instrumented` property at runtime.
+  attachBrand(wrapped, "__instrumented");
+  // Cast is the type-level concession — the brand is now also a non-enumerable
+  // runtime property attached above by `attachBrand`.
   return wrapped as Instrumented<(...args: Args) => Promise<R>>;
 }