A player can play multiple games simultaneously/ with themselves

A player, both anonymous and logged in, can play multiple games simultaneously resulting in
- playing with themselves.
- Anonymous games can not be resumed if one player is playing both games.

Start by tracing the server-side entry points that create games, assign players, and resume anonymous games. Reproduce two simultaneous games for one logged-in player and for one anonymous player. Done means a player cannot be assigned to multiple active games or matched with themselves, while anonymous games remain resumable.

Written by the indexing model from the issue text.