The board is 9 small tic-tac-toe boards arranged in a 3x3 grid. Win a small board
by getting three in a row inside it. Win the whole game by winning three small boards
in a row (a "meta" line), same pattern as regular tic-tac-toe but one level up.
Whichever cell you play in sends your opponent to the matching small board on
their turn (top-left cell sends them to the top-left board, and so on). If that board
is already won or full, they may play in any open board instead.
Only sub-boards outlined in glowing cyan are legal for your current move.