[nexus-external-api] reorganize per RFD 619

[sunshowers]

This is an extremely large PR, but a generally quite mechanical one. I had Claude Code both write most of the PR and review it for conformance with the RFD -- all tests appear to pass, and I also manually spot-checked the versions crate to ensure it generally made sense.

[sunshowers]

@david-crespo one of the things we determined in RFD 619 is that we organize types by semantic area rather than params/views/shared. This makes sense for internal APIs but would love your thoughts on this when applied to the external API (I can put things back in params/views/shared if desired, but wanted your thoughts on the complete migration).

[david-crespo]

I don't think it's a big deal either way. I'll think about it. What I like about importing views and referring to views::Disk etc. is that when you have a bunch of things all called Disk — model, view, params — it’s nice to easily tell which one you’re dealing with. On the other hand there are various other ways to tell. In this case the variable is called params, and the Create in the name also makes it clear, though that's not always the case. The type hover doesn't really tell you:

[david-crespo]

Thinking more about it, I'm fine with it. It's nice to have smaller files and it's nice to have things grouped thematically. If we're worried about confusing models and views in a particular case, we can always import with an alias or something.