Turns out heat pump fan was kicking on but compressor was not.
Technicians got compressor "bump" started but said it was "noisy and
inefficient" and producing very little pressure from the pistons.
Said they could replace it (with the physical part itself free since
it was under warranty) for $880 plus their $85 fee for the diagnostic
visit. Since the weather is nice and I don't REALLY need a/c right
now I shopped around for better prices. Eventually found a highly
rated place that would do it for $575 (plus they could do it in about
a 16 hour turnaround). I asked the the first place to give me a
better price but they dug out their files and found that we'd had a
snit about the original heat pump price three years ago (same company
supplied it; it was a York). They said based on our issues from 3
years ago they were going to stick with their current price (by far
the highest of any of my quotes).

Anyway, problem diagnosed and will be solved tomorrow. I hate to
spend the money, but I figure it's probably cheaper to have had it
happen now rather than in the middle of a heat wave when I'd have
likely been stuck paying an even higher price.