register_relay_adapter() registers the generic 'relay' platform via the same
PlatformRegistry path as plugin adapters — no core dispatch changes. OFF by
default (dark-launch): only registers when HERMES_GATEWAY_RELAY is truthy (or
force=True for tests), so existing single-tenant/direct deployments are
unaffected. Factory builds a transport-less RelayAdapter with a placeholder
descriptor; the real descriptor is negotiated at handshake.
Phase 1, Task 1.3 of the gateway-relay plan.