Well, I got it clean.
I tried the baking soda boil. That didn't touch it.
I tried ammonia. That didn't touch it.
I was at WalMart to see if they have Bar Keeper's friend and I saw... a pumice stick. A stick of actual pumice that is primarily designed for cleaning hard water stains off porcelain (toilets, tubs, and sinks).
Well, about 10 minutes of that and the pan is nice and clean.
But, as can be imagined, it's now sort of textured on the bottom compliments of the coarse nature of the pumice stick.