Merge KaraokeEvent.UsernameSet and the client-to-server KaraokeEvent.RoomDataHydration

Following #9: there are _two_ initial messages sent from a client to the server when it joins a room:

1. `KaraokeEvent.RoomDataHydration`: A request for the basic room data, made in `DynamicSceneHydrating` -> `useRoomDataConnection`.
2. `KaraokeEvent.UsernameSet`: Indicates to the server what the client username is. No response is waited for.

For the sake of simplicity & fewer messages, we should really join these two into a single one, named something like `KaraokeNite.RoomJoin`. The client can then send the username in step 1 and still hydrate room data in that same step -- instead of waiting until step 2 to set username.

Start by tracing DynamicSceneHydrating and useRoomDataConnection, then inspect the client-to-server handling of KaraokeEvent.RoomDataHydration and KaraokeEvent.UsernameSet. Confirm how the server responds to room-data hydration and how the username is stored; done means one room-join message carries the username and still hydrates room data without the separate username message.

Written by the indexing model from the issue text.