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/entities/solid3d.rs

@@ -282,6 +282,7 @@ pub fn fallback_wires(e: &EntityType) -> Option<&[acadrust::entities::Wire]> {
         EntityType::Solid3D(s) => Some(&s.wires),
         EntityType::Region(r) => Some(&r.wires),
         EntityType::Body(b) => Some(&b.wires),
+        EntityType::Surface(s) => Some(&s.wires),
         _ => None,
     }
 }
@@ -293,6 +294,7 @@ pub fn tessellate_volume(e: &EntityType, color: [f32; 4], facet_res: f64) -> Opt
         EntityType::Solid3D(s) => solid3d_tess::tessellate_solid3d(s, color, facet_res),
         EntityType::Region(r) => solid3d_tess::tessellate_region(r, color, facet_res),
         EntityType::Body(b) => solid3d_tess::tessellate_body(b, color, facet_res),
+        EntityType::Surface(s) => solid3d_tess::tessellate_surface(s, color, facet_res),
         _ => None,
     }
 }