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 <[email protected]>

src/io/mod.rs

@@ -1896,7 +1896,7 @@ pub(crate) fn is_entity_corrupt(e: &EntityType) -> bool {
             !finite_vec3(&c.center)
                 || !finite_coord(c.radius)
                 // Reject zero- or near-zero circles: they tessellate into a
-                // degenerate truck curve that crashes parameter_division.
+                // degenerate curve the tessellator cannot sample.
                 || c.radius.abs() < 1.0e-10
                 || c.radius.abs() > 1.0e10
         }
@@ -1909,12 +1909,12 @@ pub(crate) fn is_entity_corrupt(e: &EntityType) -> bool {
                 || a.radius.abs() < 1.0e-10
                 || a.radius.abs() > 1.0e10
                 // Zero-sweep arc (start_angle == end_angle, modulo 2π) collapses
-                // to a single point in WCS — truck's circle_arc on three
+                // to a single point in WCS — a three-point circle fit on
                 // coincident vertices recurses unboundedly in parameter_division.
                 || (a.end_angle - a.start_angle).abs() < 1.0e-9
                 // Near-zero sweep is the same trap with a wider mouth: a tiny but
                 // non-zero sweep (e.g. 1.6e-6 rad) still places start/mid/end
-                // within truck's coincidence tolerance, so parameter_division
+                // within the coincidence tolerance, so sampling
                 // recurses and allocates until OOM. Gate on arc *length*
                 // (radius × sweep), not sweep alone, so a legitimately large-
                 // radius small-sweep arc (still a visible curve) survives while
@@ -1924,7 +1924,7 @@ pub(crate) fn is_entity_corrupt(e: &EntityType) -> bool {
                 // the floor above, a small sweep over a modest radius leaves
                 // start/mid/end almost on one line (a 35-unit, 6.5e-7-rad arc has
                 // arc length 2.3e-5 — past the gate — yet bows off its chord by
-                // only ~2e-12). truck's 3-point `circle_arc` fit then returns a
+                // only ~2e-12). A three-point circle fit then returns a
                 // near-infinite radius and `parameter_division` subdivides without
                 // bound. Gate on the sagitta (chord height = r·(1−cos(sweep/2))),
                 // the true measure of how far the arc departs a straight line and
@@ -1951,13 +1951,13 @@ pub(crate) fn is_entity_corrupt(e: &EntityType) -> bool {
         }
         E::Spline(s) => {
             // Parser desync emits exactly-100_000-control-point splines with a
-            // garbage knot vector. Building a truck NURBS/B-spline from one and
+            // garbage knot vector. Building a NURBS from one and
             // tessellating it runs `parameter_division` into an unbounded
             // allocation — single-threaded, 32 GB+ — long before the drawing
             // finishes loading. Reject the desync signature plus any spline
-            // truck can't build: non-finite control points, or a knot vector
+            // the kernel can't build: non-finite control points, or a knot vector
             // that's non-finite, non-monotonic, or the wrong length
-            // (truck requires `knots.len() == ctrl.len() + degree + 1`).
+            // (the kernel requires `knots.len() == ctrl.len() + degree + 1`).
             let n = s.control_points.len();
             let degree_bad = s.degree < 1;
             let deg = s.degree.max(0) as usize;
@@ -1966,7 +1966,7 @@ pub(crate) fn is_entity_corrupt(e: &EntityType) -> bool {
                     || s.knots.windows(2).any(|w| w[1] < w[0])
                     || s.knots.len() != n + deg + 1);
             // Degenerate: every control point collapses onto (nearly) the same
-            // point, so the curve has zero length. truck's `circle_arc` /
+            // point, so the curve has zero length. A three-point fit
             // `parameter_division` never converges on it and the tessellation
             // hangs — a periodic 9-point spline pinned at the origin is the seen
             // case. Reject when the control-point extent is sub-precision.
@@ -2203,7 +2203,7 @@ mod corrupt_guard_tests {
     // A near-zero-sweep arc: sweep 1.56e-6 rad on a 3.9e-3 radius. The angles
     // are individually finite and the radius is in range, so the old
     // (end-start) < 1e-9 check passed it through — but start/mid/end land
-    // within truck's coincidence tolerance and parameter_division allocates
+    // within the coincidence tolerance and sampling allocates
     // until OOM. The arc-length floor must reject it.
     #[test]
     fn rejects_near_degenerate_arc() {
@@ -2218,7 +2218,7 @@ mod corrupt_guard_tests {
 
     // A 35-unit-radius arc sweeping 6.5e-7 rad has arc length 2.3e-5 — past the
     // arc-length floor — yet its start/mid/end bow off the chord by only ~2e-12,
-    // so truck's 3-point circle fit blows up and parameter_division hangs. The
+    // so a three-point circle fit blows up and sampling hangs. The
     // sagitta floor must reject it where the arc-length floor alone does not.
     #[test]
     fn rejects_near_collinear_arc() {
@@ -2246,7 +2246,7 @@ mod corrupt_guard_tests {
         assert!(!is_entity_corrupt(&EntityType::Arc(a)));
     }
 
-    // Parser desync emits 100_000-control-point splines; building a truck
+    // Parser desync emits 100_000-control-point splines; building a kernel
     // NURBS from one and tessellating it OOMs. The control-point cap rejects it.
     #[test]
     fn rejects_desync_spline() {
@@ -2256,7 +2256,7 @@ mod corrupt_guard_tests {
     }
 
     // A periodic spline whose control points all collapse onto (nearly) one
-    // point has zero length; truck's parameter_division never converges and the
+    // point has zero length; sampling never converges and the
     // tessellation hangs. The control-point extent floor must reject it.
     #[test]
     fn rejects_degenerate_point_spline() {