feat(scene): render 3D surface entities (loft/sweep/extrude/etc.)

ACAD_SURFACE entities now load and render. Their NURBS faces are
tessellated by building a truck BSplineSurface from the ACIS `spline-surface`
control net and knot vectors, then sampling its parametric grid — reusing the
same NURBS kernel the Model tab already builds on. Analytic faces
(plane/cone/sphere/torus) continue through the existing tessellator.

- New spline_tess.rs: parse the ACIS `nubs` block (degrees, knots with the
  clamped end-multiplicity fix, control grid) into a truck BSplineSurface and
  mesh it.
- tessellate_surface + Surface wired through the mesh-build, fallback-wire,
  and entity-classification paths.
- Bump acadrust to the commit that adds the Surface entity.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/helpers.rs

@@ -245,6 +245,7 @@ pub(super) fn entity_type_key(entity: &acadrust::EntityType) -> String {
         Solid3D(_) => "solid3d",
         Region(_) => "region",
         Body(_) => "body",
+        Surface(_) => "surface",
         Mesh(_) => "mesh",
         Ray(_) => "ray",
         XLine(_) => "xline",