I ran a
sudo apt upgrade, rebooted, and now I can't log in.
The odd thing is this... when I provide known invalid credentials the login dialog reports 'Incorrect password' in a red block, like so:
... but when I enter my known correct credentials the dialog disappears entirely for a moment and then returns with the password field empty (no red block).
Unfortunately I didn't enable SSH on this one, and have been connecting solely via VNC (lesson learned).
FYI the RPi is local, on the same LAN—if that helps.
Am I out of luck? Will I have to reinstall from scratch? (I sure hope not.)