Declarative constructed solid geometry sounds like how OpenSCAD works. I was curious if you took any inspiration from that project, or if you had found it but didn't suit your needs for some reason...
They don't. So save yourself that trouble. You design the fillets right into the extrusions doing them after the fact is prohibitively expensive.
They have to be done manually, usually using the Minkowski feature iirc.
There's another similar tool called implicitcad that handles them better (it's also the only useful piece of software written in Haskell I've ever encountered) https://implicitcad.org/
pandoc and xmonad are super useful