feat(model): SLICE cuts meshes

SLICE now takes MESH, polygon mesh and polyface mesh objects. A mesh
that closes a volume is sliced as a solid, so each side is capped and
becomes a 3D solid. An open mesh, such as a pipe run imported without
end caps, is clipped face by face and each side stays a mesh, open
along the cut. Keeping one side or both works as for solids, in one
undo step.

Refs #1398

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

src/modules/model/slice_cmd.rs

@@ -335,8 +335,15 @@ impl CadCommand for SliceCommand {
         self.selected = entities
             .into_iter()
             .filter_map(|item| {
-                matches!(item.entity, EntityType::Solid3D(_) | EntityType::Surface(_))
-                    .then_some(item.handle)
+                matches!(
+                    item.entity,
+                    EntityType::Solid3D(_)
+                        | EntityType::Surface(_)
+                        | EntityType::Mesh(_)
+                        | EntityType::PolygonMesh(_)
+                        | EntityType::PolyfaceMesh(_)
+                )
+                .then_some(item.handle)
             })
             .collect();
     }