fix: write each lump of a body as its own STEP solid (#455)

A body of several pieces (a plate and the islands a cut leaves inside its
holes, such as letter counters) was written as one BREP_WITH_VOIDS whose
other shells were voids. Export now writes one solid per lump, each with
its face colours; a real void stays a void of its lump.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/kernel/src/body_lump_tests.rs

@@ -115,7 +115,9 @@ fn curved_internal_void_and_disconnected_material_keep_analytic_volume() {
     }
     let imported = step_import(&step_export(&[&body], "curved nested cavities").unwrap()).unwrap();
     assert!(imported.warnings.is_empty(), "{:?}", imported.warnings);
-    let imported: Vec<_> = imported.bodies.into_iter().flat_map(|b| b.body.lumps().unwrap()).collect();
+    // The two pieces are written as two solids (each with its own void), so they come back as
+    // two bodies.
+    let imported: Vec<_> = imported.bodies.into_iter().map(|b| b.body).collect();
     assert_eq!(imported.len(), 2);
     let imported_meshes: Vec<_> = imported.iter().map(|b| b.tessellate_with(0.01, true).unwrap()).collect();
     for (p, want) in [(Vec3::ZERO, 0), (Vec3::X * 3.0, 1), (Vec3::X * 7.0, 0), (Vec3::X * 15.0, 1)] {