Hah, I actually wrote a similar constrain solver that uses Fidget's evaluation + differentiation! This blog post was getting too long, so I'm going to write it up separately, but in the meantime:

demo: https://mattkeeter.com/projects/fidget/constraints

source: https://github.com/mkeeter/fidget/blob/main/demos/constraint...

docs on the solver: https://docs.rs/fidget/latest/fidget/solver/

You might be interested in http://omrelli.ug/g9/gallery/ too