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/doc/src/document.rs

@@ -1475,6 +1475,11 @@ pub enum FeatureKind {
         /// The file's product/assembly tree, kept for components.
         #[serde(default, skip_serializing_if = "Vec::is_empty")]
         components: Vec<solidcraft_kernel::StepNode>,
+        /// The file's bodies (by index) this feature does not build: later instances of a part
+        /// the file places more than once, shown by the other occurrences of that part's
+        /// component (#1386). Empty in designs from before format 22: every body is built.
+        #[serde(default, skip_serializing_if = "Vec::is_empty")]
+        skip: Vec<u64>,
     },
     /// Rebuild an upstream constant-section mesh with analytic planar/cylindrical faces.
     MeshConvert {