More info. I am changing the radiator fluid so I have to do many cycles of drain, refill, run car to warm up, cool down, drain, refill...
During one of these repetitions last night, I sprayed a tiny amount of belt dressing on the ribbed side of the belt. Now instead of chirping once in a while, it squeals non-stop at idle, and doesn't quiet down until the rpms go up over about 1500. This would seem to point to the belt vs. the water pump, but the belt visually seems to be in great condition. I used a mechanic stethescope to listen to all the bearings and the tensioner bearings are definitely grumbly, but the don't seem to be the cause of the squealing. It's really really hard to isolate the source of the noise which is frustrating. I might just try ordering the belt and the two pulleys today and seeing if that stops it.