Defines RelayTransport (lifecycle/handshake/inbound/outbound/interrupt) as the
gateway<->connector wire contract; RelayAdapter.connect now registers an inbound
handler that bridges connector-delivered MessageEvents into handle_message.
Adds an in-memory StubConnector under tests/ and an E2E round-trip proving:
connect registers the handler, inbound events reach the adapter, guild_id drives
build_session_key isolation (two guilds -> two keys; same guild/channel/user ->
one), outbound send round-trips, get_chat_info is proxied.
Phase 1, Task 1.2 of the gateway-relay plan.
