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/config_certificate_tests.rs

@@ -73,7 +73,7 @@ fn imported(session: &mut Session, step: String, name: &str) -> u64 {
     let owner = session.active_component;
     let id = session
         .doc_mut()
-        .add_feature(FeatureKind::Import { file: "fixture.step".into(), step: step.into(), components: vec![] }, Some(name))
+        .add_feature(FeatureKind::Import { file: "fixture.step".into(), step: step.into(), components: vec![], skip: Vec::new() }, Some(name))
         .unwrap();
     let feature = session.doc_mut().feature_mut(id).unwrap();
     feature.body_names = vec![name.into()];