Same treatment the model/thinking controls got (df4433f): the
per-turn stats popover now closes on any pointerdown outside the
bar and on Escape. Also centered over the stats bar
(left-1/2 -translate-x-1/2) instead of left-aligning with the quiz
box, which looked odd.
Verified live: popover centered over the bar (centers within 2px);
Escape closes; click outside closes; click inside stays open.