Jets: one shared assignment check for dense and fine levels

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>

tests/unit/test_jets.py

@@ -276,3 +276,10 @@ def test_fine_boundary_keeps_edge_frame_hits_and_checks_jets():
         refinement.prepare_fine_boundary(cfg, patch, [body], jets=[jet, jet])
     with pytest.raises(ValueError, match="need fine boundary objects"):
         refinement.prepare_fine_boundary(cfg, patch, [], jets=[jet])
+    # Jet vs legacy SurfaceActuation overlap on the fine level (root's case).
+    legacy = SimObject(0, geometry=CircleGeometry(*C, R), center=C, radius=R,
+                       surface_actuation=SurfaceActuation(0., .5, .002))
+    with pytest.raises(ValueError, match="SurfaceActuation"):
+        refinement.prepare_fine_boundary(cfg, patch, [legacy], jets=[jet])
+    disjoint = Jet(Sector(C, math.pi, .5, R + .01), Radial(C, inward=False))
+    refinement.prepare_fine_boundary(cfg, patch, [legacy], jets=[disjoint])