I've added a new level select scene after picking your set, and I've
updated the engine to store the result of each puzzle separately. The
level select is paged (can't fit all 50 on one screen), so needed to add
a new "previous button" graphic.
Added confirmation prompts, so I can prompt before quitting, and for a
new menu option to clear your scores if you want to reset your progress.
Progress is still not persisted when you exit, as I haven't figured out
how to do that yet.
I've also rearranged the title screen a bit.