Offtopic shoutout for solvespace, it is amazing! Is separating out the geometry code into a stand alone library on the roadmap at all? Or plans to incorporate the manifold library?
If you mean the NURBS code, there are no plans to separate it for now. It does live almost entirely in the src/surf folder and does not depend on the sketch entities or constraint solver.
My hope is to squash the remaining bugs in that code. I'm currently working on an issue with coincident and tangent surfaces.
The constraint solver is available as a library and is currently used in FreeCAD assembly 3 workbench, as well as the Blender CAD Sketcher addon.
https://github.com/solvespace/solvespace/issues/853