Throwing in the towel. My solution works for the test input.

I get this message for the answer: "Curiously, it's the right answer for someone else; you might be logged in to the wrong account or just unlucky. In any case, you need to be using your puzzle input."

This post is part of my Freewheeling Apps Devlog.

Comments gratefully appreciated. Please send them to me by any method of your choice and I'll include them here.