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/format.rs

@@ -200,13 +200,21 @@
 //!   and a bend ending beside more of the fixed face its bend relief (0.11.0 left every corner a
 //!   notch and cut no bend relief), and a wall on part of an edge bends only where it stands
 //!   (0.11.0 refused it).
+//! - 22: an Import (STEP) feature can list `skip`: indices of the file's bodies it does not
+//!   build, because they are later instances of a part the file places more than once and the
+//!   other occurrences of that part's component show them (#1386: an imported file becomes a
+//!   component, an assembly in it nested components). Version 21 designs need no
+//!   transformation: an Import without it builds every body, as it always did, and designs
+//!   whose imports sit loose in a component are left as they are. The version stamp makes
+//!   0.12.0 (which reads up to 21) ask for an update instead of building those instances again
+//!   on top of their occurrences.
 
 use serde_json::Value;
 
 use crate::{DocError, Document, Result};
 
 /// The format this build writes.
-pub const FORMAT: u32 = 21;
+pub const FORMAT: u32 = 22;
 
 pub fn format_string(n: u32) -> String {
     format!("solidcraft/{n}")
@@ -223,7 +231,7 @@ pub fn version_of(s: &str) -> Option<u32> {
 
 /// JSON-level upgrades: entry `i` turns format `i + 1` into `i + 2`.
 type JsonStep = fn(&mut Value) -> Result<()>;
-const JSON_STEPS: [JsonStep; 20] = [
+const JSON_STEPS: [JsonStep; 21] = [
     |_| Ok(()),
     v2_to_v3,
     v3_to_v4,
@@ -244,6 +252,7 @@ const JSON_STEPS: [JsonStep; 20] = [
     |_| Ok(()),
     |_| Ok(()),
     |_| Ok(()),
+    |_| Ok(()),
 ];
 
 /// Stored face and edge names were worked out from geometry before format 14; they get the