I edited my last post.
This seems the likely culprit. I’ve been clearing disk space and upgrading via apt, and after a reboot, I built again and it actually completed. Although there were package changes, I think the reboot restored just enough ram/swap to let the build finish. Interestingly, I’ve had to stop upgrading via the UI because of low memory (I exclusively do it from the command line), but I’ve never had a build fail at the command line. I’ll see about increasing my swap another 1 GB to prevent this in the future and see if I can restore my ability to upgrade via the UI.