I left straightforward comments on GitHub for the ap_pipe docs, and pending those, it looks good. I'll take the liberty of marking this as "reviewed" for all three of us.

One somewhat larger question - is the Gen 3 implementation done here kind of an intermediate step while we still need to support Gen 2? It's my vague understanding that eventually, running the AP Pipeline will just be having a yaml file with "do ISR, do characterization, do calibration, do differencing, do association, the end" and we won't need an ap_pipe package to link those steps together. Maybe something to discuss at our weekly meeting if I'm off base. For now, I'm thrilled it works!

What about self.metadata? What was the reason for the introduction of algMetadata?

I don't know anything about algMetadata, including whether or not it's related to metadata (though its attachment to source catalogs would make me think it's something different). Since Gen 3 tasks are supposed to be stateless, I assume metadata is going away at some point, but last I checked Middleware didn't have a plan for a replacement yet.

It's my vague understanding that eventually, running the AP Pipeline will just be having a yaml file with "do ISR, do characterization, do calibration, do differencing, do association, the end" and we won't need an ap_pipe package to link those steps together

I think as long as we have ApPipeTask, it would be confusing for users to need different packages depending on whether they're running the pipeline in Gen 2 or Gen 3. So I'd be opposed to moving to pipe_tasks before we phase out ApPipeTask. We'd also need to do something about make_apdb or its replacement (see DM-22663); maybe put it in ap_association?

What is this metadata that is being discussed?

One is the standard Task.metadata. The other is ImageDifferenceTask.algMetadata, which seems to be associated with measurement algorithms in some way.