Freeze the public relay-server contract #67

Question

After the compatibility, cardinality, reachability, byte-limit, and termination decisions are resolved, what exact public contract should the four-PR specification expose? Freeze RelayServerConfig, rate-limit configuration, RelayServerEvent, close reasons, Endpoint builder methods, feature composition, runtime enablement if any, re-exports, error boundaries, and the complete list of documented deviations, while keeping stream-level plumbing internal.