- #pragma pack(push,1) on the wire structs (the relay's proto is packed; the
transport's were not, which would inflate sizes and corrupt every datagram)
plus static_asserts pinning sizes to 7/31/1410/25/801.
- Heartbeat re-JOIN every 4s so the relay never times the transport out.
- REXLOG info for connect/join/session-reply so the flow is observable.
