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>

src/modules/draw/modify/offset.rs

@@ -21,6 +21,7 @@ use acadrust::entities::{
 use acadrust::{EntityType, Handle};
 // Polyline offsetting, and the angle normalisation that goes with it, come
 // from the kernel; only the entity conversion stays here.
+use acadrust::kernel::geom2d::nurbs::clamped_uniform_knots;
 use acadrust::kernel::geom2d::{
     normalize_angle as norm_rad, offset_polyline, Polyline as KernelPolyline,
     PolylineVertex as KernelVertex,
@@ -297,11 +298,10 @@ fn offset_spline(spl: &SplineEnt, dist: f64, side_pt: Vec3) -> Option<EntityType
     let degree = spl.degree.max(1) as usize;
     let new_ctrl: Vec<acadrust::types::Vector3> = offset_pts;
     let n_ctrl = new_ctrl.len();
-    let kv = truck_modeling::KnotVec::uniform_knot(degree, n_ctrl - 1);
     let mut new_spl = spl.clone();
     new_spl.common.handle = Handle::NULL;
     new_spl.control_points = new_ctrl;
-    new_spl.knots = kv.iter().copied().collect();
+    new_spl.knots = clamped_uniform_knots(degree, n_ctrl);
     new_spl.fit_points.clear();
     new_spl.weights.clear();
     Some(EntityType::Spline(new_spl))