I know that updates were not installing when I rebooted. It may have been checking for updates, but it definitely wasn't installing them. To recover, I booted

I know that updates were not installing when I rebooted. It may have been checking for updates, but it definitely wasn't installing them. To recover, I booted from an installer image on a USB drive and tried to mount the partition R/W and it hung. Since the installer is a known-good environment, this rules out breaking my system the way you describe. This was a corrupt BTRFS filesystem. One possibility is that because I was using a rolling-release distribution, I may have gotten a version of BTRFS with a bug in it or minor changes in the BTRFS code as the system got updated eventually got the filesystem into a state that rendered it unable to mount R/W. Either scenario doesn't inspire confidence.