Add ConcurrentDictionary<ulong, RouteConnection> _routesByHash with FNV-1a 64-bit hash key, RegisterRouteByHash/UnregisterRouteByHash/GetRouteByHash/ GetRouteByServerId methods, and HashedRouteCount property to RouteManager. Includes 10 unit tests covering determinism, distinct hashes, CRUD lifecycle, overwrite semantics, and no-op unregister.