Refactor the stop case in Hub.Run() to iterate h.rooms directly
instead of h.connRoom. For each room, iterate all connections and
send CloseGoingAway frame before closing. After the loop, reset both
maps (h.rooms, h.connRoom) in one shot rather than deleting entries
incrementally. This is cleaner and avoids modifying a map during
iteration.
Add TestIntegration_GracefulShutdownMultiRoom to verify clients in
separate rooms all receive close frames during shutdown.
🤖 Assisted by the code-assist SOP
c226bdab7f