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>

docs/book/src/guide/files.md

@@ -16,6 +16,13 @@ Opening a STEP or IGES file creates a design with the file's bodies as a base fe
 inserting one adds them to the current design. Later features build on them. Mesh bodies render,
 measure, move and export, but solid features need B-rep bodies.
 
+Every imported file (STEP, IGES, 3MF, STL or OBJ) comes in as a new component named after the
+file, so its bodies never mix with yours. A STEP assembly keeps its structure: its parts and
+sub-assemblies become nested components, and a part used several times is one component with an
+occurrence for each place. The imported assembly is rigid: moving its component moves everything,
+and its parts don't move on their own. Right-click the component and choose **Unlock Contents** to
+joint the parts one by one. Undo removes a whole import in one step.
+
 **File → Save as mesh…** (also on the Mesh tab under Modify, and in the right-click menu of a
 body) saves the selected bodies as STL, 3MF or 3MF with face tags (below). With **Send to slicer** ticked, it writes the file to
 a temporary folder and opens it in the chosen slicer instead of asking for a file name. The slicer