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/tests.rs

@@ -307,6 +307,7 @@ mod blend_fit;
 mod boolean_diagnostics;
 mod component_frames;
 mod curved_seam_round;
+mod import_component;
 mod large_step_import;
 mod loft_cut;
 mod loft_ring;
@@ -1057,7 +1058,9 @@ fn join_onto_a_finely_faceted_import() {
     let prism = volume(&mut s);
 
     let mut d = Session::default();
-    run(&mut d, "file.insert_step", json!({"path": p.to_string_lossy()}));
+    // The import is a component (#1386): the pin is made in it to join its body.
+    let r = run(&mut d, "file.insert_step", json!({"path": p.to_string_lossy()}));
+    run(&mut d, "component.activate", json!({"component": r["component"]}));
     run(&mut d, "sketch.create", json!({"plane": "XY", "name": "Pin"}));
     run(&mut d, "sketch.rectangle.two_point", json!({"p0": [-5, -5], "p1": [5, 5]}));
     run(&mut d, "sketch.finish", json!({}));