Auto-shuffles the seats at game start when enabled (now for games with
two or more players instead of three or more) and handles the new
SET_PLAYER_ORDER command so the game host or a moderator can reorder or
randomize the seats before the game starts. Re-seating re-keys the
player ids while keeping the per-user game/seat routing and persistent
player records in sync, then broadcasts the new order to everyone.
