Require integrity protection for MRTR requestState (#3032)

examples/stories/refund_desk/README.md

@@ -29,7 +29,9 @@ uv run python -m stories.refund_desk.client --http
 - `server.py` `refund_order` — the signature is the whole story: `order_id` and
   `reason` are model-facing; `cents` and `restock` carry `Resolve(...)` markers
   and never reach the input schema. `client.py` asserts `properties` and
-  `required` are exactly `{order_id, reason}`.
+  `required` are exactly `{order_id, reason}`. At 2026 the resolver's elicited
+  answers ride between rounds inside a `requestState` the SDK seals by default;
+  see `mrtr/` for the full security walk-through.
 - `server.py` `refund_scope` — the no-round-trip fast path: a one-line order
   returns `Scope(full=True)` directly; only a multi-line order returns
   `Elicit(...)`. The ORD-7001 call completes with zero elicitations.