First: thank you, enziosavio and karlmistelberger, for your responses.
This problem has been fixed, and I can only scratch my head on why and how it occurred in the first place.
I keep a virtual Tumbleweed machine on disk for testing purposes, and decided to zypper dup this VM to the 20181219 snapshot to see if it would suffer similar symptoms. It didn’t. I shrugged, decided to try again on the bare metal install, and again zypper dup’d from 20181218 to 20181219.
The second time must have been the charm. Now everything works. No hangs in zypper dup, no hangs in Yast → Software Management. I did nothing differently. Perhaps Christmas hating pixies were at work, and Santa came to my rescue. <g>