3MF: export, and 3MF/STL import as mesh bodies

Export: ExportCommand/CLI --out/MCP export with .3mf write a 3MF Core package
([Content_Types].xml, _rels/.rels, 3D/3dmodel.model; millimetres), one
mesh object and build item per body, named after it, vertices shared and
welded watertight (T-junctions between faces split), base-material colours
when bodies carry one (STEP colours now ride on bodies). Tessellation as
for STL. File > Export 3MF.

Import: doc.open/drag-drop/CLI/MCP open accept .3mf and .stl; Insert Mesh
(ParaMeshInsertAlignCommand) adds them to a design as a MeshImport feature.
Kernel mesh bodies render, measure exactly, move and export; solid
features refuse them with a clear error. 3MF reading flattens components
and build transforms, converts units, reads colours; sizes, entries,
nesting and element counts are capped.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/kernel/src/step_in/mod.rs

@@ -455,14 +455,8 @@ impl<'a> Reader<'a> {
                             let own = e.params().first().and_then(Param::as_str).unwrap_or("").trim().to_string();
                             let name = if own.is_empty() { product.to_string() } else { own };
                             out.push(self.bodies.len());
-                            self.bodies.push(ImportedBody {
-                                name,
-                                body,
-                                color: self.colors.get(&item).copied(),
-                                path: path.to_vec(),
-                                closed,
-                                file_faces,
-                            });
+                            let color = self.colors.get(&item).copied();
+                            self.bodies.push(ImportedBody { name, body: body.with_color(color), color, path: path.to_vec(), closed, file_faces });
                         }
                         Err(err) => {
                             let label = self.label(item);