Move surface_link_velocity (pure host code) from lbm/curved_links.py to
body/jets.py, re-exported by curved_links, so refinement.py can reuse it
without importing PyCUDA. prepare_fine_boundary now runs the same
assignment validation as the steppers when jets are given, rejecting
jet-vs-SurfaceActuation overlap (root's C(12.25,12.25) R2.15 case) as
well as empty/overlapping jets. Test the overlap and a disjoint pair.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>