Thanks everyone for the advice!
I took out the wheel and unclamped the shifter cable from the derailleur, and quickly found the problem. The cable was not sliding freely through the last piece of housing, either when pulled by the shifter or by hand. Looking more closely at the housing, I can see a spot where it looks like it buckled under the compression, and when I try to pull the cable through by hand, it grinds at that spot.
Looks like I have a nice walk to the shop ahead of me this morning!
All praise be to Allah, your cable's onset of failure occurs when the bike is in your workshop instead of out on the road.
Yes indeed! Thanks again.