feat(section): volume top/bottom grips; live sections cut closed meshes

A section volume now shows grips on its top and bottom faces, so its
height is pushed and pulled in place like its sides, not only through
the Properties palette. Live sectioning also cuts MESH, polygon mesh and
polyface mesh objects that close a volume, drawing each as the capped
solid it bounds in every visual style.

Refs #1407

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/entities/extended.rs

@@ -1971,6 +1971,12 @@ fn grips(entity: &ExtendedEntity) -> Vec<GripDef> {
                     })
                     .collect()
             };
+            // A section volume is also pushed or pulled at its top and bottom.
+            if data.state == 4 {
+                grips.extend(section_plane_edge_grips(data).into_iter().filter(|grip| {
+                    matches!(grip.id, SECTION_GRIP_TOP | SECTION_GRIP_BOTTOM)
+                }));
+            }
             if let Some(right) = data.vertices.last() {
                 let vertical = normalized(data.vertical_direction, Vector3::UNIT_Z);
                 let edge_center_offset = (data.top_height - data.bottom_height) * 0.5;

src/scene/mod.rs

@@ -8847,6 +8847,14 @@ impl Scene {
                 body.wires = wires;
                 body.silhouettes.clear();
             }
+            // The cut mesh is drawn as the solid it bounds.
+            EntityType::Mesh(_) | EntityType::PolygonMesh(_) | EntityType::PolyfaceMesh(_) => {
+                let mut solid = codec::entities::Solid3D::new();
+                solid.common = entity.common().clone();
+                solid.point_of_reference = reference;
+                solid.wires = wires;
+                entity = EntityType::Solid3D(solid);
+            }
             _ => return Err(()),
         }
         Ok(Some(entity))
@@ -8872,6 +8880,12 @@ impl Scene {
             Some(EntityType::Surface(surface)) => {
                 crate::scene::convert::solid3d_tess::kernel_surface_body(surface)
             }
+            // A mesh that closes a volume is cut as the solid it bounds.
+            Some(
+                mesh @ (EntityType::Mesh(_)
+                | EntityType::PolygonMesh(_)
+                | EntityType::PolyfaceMesh(_)),
+            ) => crate::entities::mesh::closed_mesh_body(mesh),
             _ => None,
         }?;
         Some(std::borrow::Cow::Owned(body))