In my first run tonight, I soldered everything without the amp on because I was confident I knew what to connect. When I turned the amp on, I got a loud buzzing that sounded like something was disconnected. I undid my pickup soldering and still had the buzz.
I thought I had somehow broken the volume potentiometer because it was still going after turning it all the way down. (I should have picked up on this clue.)
Then, I disconnected the potentiometer. Still buzzing.
So, the ground connection broke right at the jack, which I should have checked first.