Split out fgensolving or similar

The problem

A bunch of the stuff in here is stuff that we actually want to wrap with fgen, rather than being core fgen stuff itself. For example, all the solving stuff. Having them in the same repo is painful for maintenance but also conceptually.

Definition of "done"

• split out a repository that uses fgen to wrap things that contains our model solving stuff, this could be called e.g. fgensolving
• tested the linking/vendoring process for combining fgensolving with an actual model implementation (this could be non-trivial)

Additional context

There are lots of edge cases to consider here, particularly for the linking. This conversation makes a good start listing some of them: !48 (comment 1758485433)

This issue may have to be split into two/moved to other repositories. It is a good placeholder/reminder for now.