Hot Water Supply Line Stopped
The solution turned out to be disconnecting and reconnecting the inlet side of the supply line, because -- here's the kicker -- the supply line is a Floodsafe line designed to automatically shut off completely if the flow rate exceeds a preset maximum, which I guess I exceeded momentarily. I didn't see the labeling on the supply line until after I disconnected it, and this is the first time it happened in 1.5 years of living here. Frankly, I don't see the advantage of this. It won't do anything to stop common slow, drippy leaks.