feat(client): ISession.rename settles the title projection cell from the response

rename is a per-session verb on the outward session face (prompt/cancel
precedent), not a list-service verb: the Session calls session.rename and
applies the response {title, seq} to its projection store under
higher-seq-wins, so the list row updates before the push frame. The fixture
api and the test-runtime double follow the same face.

packages/client/test-runtime/src/sessions.ts

@@ -107,6 +107,14 @@ export class FixtureSession implements SessionFace {
   loadOlder(): never {
     throw new Error(`test session "${this.sessionId}": loadOlder is not stubbed — supply it on the fixture's session face`)
   }
+
+  /**
+   * Fail-loud stub; supply `rename` on the fixture's session face to exercise it.
+   * @returns never — always throws.
+   */
+  rename(): never {
+    throw new Error(`test session "${this.sessionId}": rename is not stubbed — supply it on the fixture's session face`)
+  }
 }
 
 /** One live test session: fixture-derived stores plus its minted scope state. */