refactor: drop truck, and evaluate every curve through the kernel

The last four truck crates are gone from the manifest. What they were doing
falls into three parts, and each now has one answer instead of two.

NURBS evaluation. truck was carrying B-spline curves and surfaces for the
SPLINE entity, hatch boundaries, BLEND's endpoint frames, the spline preview
and ACIS spline-surface faces. The kernel now has both, over one de Boor
written across however many coordinates a control point holds — so a plane
curve and a space curve cannot drift apart, and a surface is the same
algorithm applied twice. The rational and the polynomial cases stop being
separate types: weights absent means polynomial.

The entity conversion. Every LINE, ARC, ELLIPSE, SPLINE and polyline was
built into truck topology purely so it could be sampled back into points.
They are sampled through `entities::curve` now, which is where each one's
geometry is already defined once and what EXTRUDE and REVOLVE read — so a
circle drawn on screen and a circle handed to the Model tab come from the
same definition. That retires four of TruckObject's variants and the module
is renamed for what it does.

SWEEP and LOFT. Both only ever produced a mesh, so both are built from point
lists: a band of quads per span, and a lid where a profile closes. Lofting
profiles of different densities resamples them by distance rather than by
index, so a circle lofted to a square no longer twists.

Two things worth noting for anyone reading the old comments. The tolerance
globals that lived in the tessellation module were never about truck at all —
they are the per-frame chord height, and they move to `curve_tol`. And the
rule that a profile handed over as `Lines` silently broke EXTRUDE and REVOLVE
no longer holds: those read the curve directly and never look at this channel.

The two `automation` test failures are unchanged from before this and are not
caused by it.

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

src/modules/insert/solid3d_cmds.rs

@@ -1,7 +1,7 @@
 // 2D→3D modelling commands — EXTRUDE, REVOLVE, SWEEP, LOFT.
 //
 // Each picks profile/path entities and emits a `CmdResult` whose handler
-// builds a truck solid and inserts its MeshModel into scene.meshes under a
+// builds a solid and inserts its MeshModel into scene.meshes under a
 // minimal placeholder Solid3D entity (empty ACIS — saving will not restore
 // the mesh). The standalone primitives (BOX/CYLINDER/CONE/SPHERE/WEDGE/TORUS)
 // live in the Model tab (`modules::model::primitive_cmd`).
@@ -13,7 +13,7 @@ use crate::t;
 use crate::command::{CadCommand, CmdResult};
 
 // BOX / SPHERE / CYLINDER (and CONE / WEDGE / TORUS) now live in the Model tab
-// (`modules::model::primitive_cmd`), which builds them as truck B-reps cached
+// (`modules::model::primitive_cmd`), which builds them as B-reps cached
 // for the Design-group boolean tools. EXTRUDE / REVOLVE / SWEEP / LOFT remain
 // here as 2D→3D operations.