refactor(spline): evaluate splines with cadkernel, weights and all

The spline path ran on truck's BSplineCurve<Point3>, which is not
rational. Weights were dropped on the way in and cleared on every split,
so a NURBS circle — which is how most circles arrive once they have been
through a modeller — was evaluated as a parabola and stayed one after a
break. cadkernel's NurbsCurve carries the weights and applies them in
homogeneous coordinates, and `spline_cut` splits by raising the cut to
full knot multiplicity, so both halves are exact and both stay rational.

`spline_ops` keeps the knot-domain call shapes the commands were written
against — `spline_range` replaces reaching for `range_tuple`, `spline_cut`
replaces `cut` plus the rebuild — so TRIM, BREAK, LENGTHEN and OFFSET
changed only where they named those.

Knot vectors for a rebuilt control polygon come from the kernel too, so
the clamped uniform vector is defined once rather than assumed in two
places.

truck is now absent from the whole spline path. It remains for 3D solids.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Cargo.lock

@@ -945,7 +945,7 @@ checksum = "fc652a48c352aef3ea3aed32080501cf3ef6ed5da78602a020c991775b0aff04"
 [[package]]
 name = "cadkernel"
 version = "0.1.0"
-source = "git+https://github.com/HakanSeven12/cadkernel.git#feed535a0f72f9479d618a4999dba9bafcea250f"
+source = "git+https://github.com/HakanSeven12/cadkernel.git#33aa850ac2a27fdb2b305a67df27cb0eae23acc2"
 dependencies = [
  "cavalier_contours",
 ]