Snake: speed derived from pixels/second; +1 px/sec per 5 food eaten
(pps = 70 + food/5, step = cell*1000/pps, floored at 40ms).
Tetris: speed derived from pixels/second; +1 px/sec per 10 lines cleared
(pps = 23 + lines/10, step = CELL*1000/pps, floored at 40ms), replacing
the old level-driven formula. Level still drives the score multiplier.
Tetris: list all controls concisely in the free space of the right panel
(KEYS, 4/6 MOVE, 2 ROTATE, 8 SOFT, 0 DROP, # PAUSE, 5 OK).
Luxferre
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