feat(examples): fold next/chats into next/routing on a plain Durable Object hub reached with useAgent (#2246)

The chats example becomes examples/next/routing: a plain UserHub DurableObject composed with RoutedAgents and WebSockets, its RpcTarget reached from the React sidebar through useAgent on either transport, one ChatAgent per chat behind the hub's route. Push-back index, ordering fence, and delete semantics unchanged. Runtime validation on browser-controlled arguments, shared message bounds in a dependency-free module, and docs plus the catalog now point at next/routing.