Adds GatewayConnectionState enum, GatewayRegistration record with atomic
message counters, and a full registry API on GatewayManager: RegisterGateway,
UpdateState, GetRegistration, GetAllRegistrations, UnregisterGateway,
GetConnectedGatewayCount, IncrementMessagesSent, IncrementMessagesReceived.
Covers 11 new tests in GatewayRegistrationTests.cs (all passing).
