A plugin that calls flag.leave() on `open` (Tag and Infected outside a session) sent the game an
exit from a setup that never opened. The game never took it, and every later flag's setup waited
behind it. A registration left before its setup was asked for now just ends, from flag.leave()
and from the waiting card's Leave alike (leave_unopened).
The join card says which flag it read and how it was branded, or which buttons it has when it has
no Enroll; its branding errors were swallowed. Joining another player's plugin flag says whether
its plugin heard join, or why the player was taken out of its queue. A navigation that waits on
the game's says so every 3 s, and a setup that does not open in 15 s says so.