Imports become components; STEP assemblies as nested components (#1386) (#1426)

* Imports become components: a STEP assembly's tree as nested components (#1386)

Every imported file (STEP, IGES, 3MF, STL, OBJ: inserted, dropped, opened as a
design, or file.insert_step / file.insert_mesh / doc.open from scripts, the CLI
and MCP) now comes in as a new component named after the file, with its Import
feature inside it. A STEP assembly's products become nested components; a part
placed more than once is one component with an occurrence per placement, and
the Import feature skips the file's later copies (`skip`, format 21 note). The
contents are held as one rigid group (rigid joints, glyphs hidden); moving an
inner part is refused until component.unlock ("Unlock Contents") releases it.
Y up turns the import's component. Replies report component, component_name
and occurrence. Undo is one step; designs with loose imports are unchanged.

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

* Keep an import's rigid group hidden when the window loads the file in the background

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

* Import instances' skip list is format 22: format 21 ships in 0.12.0

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

* Catalog entries for Unlock Contents

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

---------

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

crates/engine/src/cmd/component_clone.rs

@@ -346,7 +346,7 @@ pub(super) fn paste(
         bodies.insert(body.name.clone(), new_name.clone());
         let feature_name = unique(&format!("{} Base", body.name), &mut used);
         let id = scratch.doc_mut().add_feature(
-            FeatureKind::Import { file: format!("{} independent base", body.name), step: step.into(), components: Vec::new() },
+            FeatureKind::Import { file: format!("{} independent base", body.name), step: step.into(), components: Vec::new(), skip: Vec::new() },
             Some(&feature_name),
         )?;
         if let Some(f) = scratch.doc_mut().feature_mut(id) {