Verify prototype overlay boundaries

examples/library-adoption/README.md

@@ -36,6 +36,11 @@ from `profiles/`. An explicitly supplied `--proposal-profile` or
 requested overlay and each effective profile, backend, and model. Synthetic model
 IDs demonstrate operator-selected routing; they are not measurements of price,
 speed, or model quality.
+An overlay can also change ordinary backend settings and credential references.
+ConfigKit resolves only the surviving reference at a replaced credential path;
+changing role profiles alone leaves other configured backend credential references
+in place, and unresolved references still fail loading. Explicit `null` credentials
+select unauthenticated backend use.
 
 Tests control provider responses, capacity, and cancellation. They cover positive
 repair budgets for both roles, exhausted repair, domain rejection, transient retry,