Verify an A* heuristic by diffing node expansions

A cheap, decisive correctness check for an admissible heuristic: run the same problem with and without the heuristic. A correct admissible heuristic yields the identical optimal path length while expanding strictly fewer states. On an open grid this showed 636 vs 2728 states explored for the same length-55 path — confirming both optimality and that the heuristic is actually doing work.