a2aCheckout over A2A: the profile adds an A2A service whose agent card is at /.well-known/agent-card.json; checkout runs over A2A 0.3 (message/send) or 1.0 (SendMessage) at /ucp/a2a with the UCP extension. REST and MCP stay, and catalogue search stays on them. on flowers-a2a.ucpsandbox.com. The baseline store is flowers.ucpsandbox.com.