v1.12

Improved final puzzle, and lots of small tweaks:

• Puzzle 12 redone to be more... readable? (No change to the end result.)
• Grid 0 is less ambiguous. Also, "7?" -> "Try 7 or 9".
• Added letters to bottom-right corners.
• Grids 1 and 5 are slightly easier.
• Grid 11 redone to be harder, with obscured corners. Also, "20" -> "a8".