More news. I pulled the head, and it looks perfect. I took it to my head shop (not the one with bongs) and he pulled a vaccuum on every port and the valves were sealing nicely. That was good news in a way. I still have no idea why I can't build compression, though I am ruling things out one at a time.
The only things left questionable are the rings. The motor has a fresh bore and hone. The rings came from the piston manufacturer and were filed to fit? Maybe, can't remember. I did measure the gaps at least and wrote down that the top and middle were both .018" gaps when installed. They might have been really close out of the box.
Could they be installed upside down? Wrong ring in the top groove? Would I get this problem by doing something silly like this? Or would it be somewhat normal for a motor that has never been fired leak this much? Doubtful.