feat: save body and occurrence visibility with the design (#531) (#592)

Hidden bodies, occurrence placements, sketches and origin items are saved with the design and come back when it is reopened. Visibility is still not an undo step, but hiding or showing something now counts as an unsaved change. Additive field under format 16. Fixes #531.

Co-authored-by: jonaswre <jonas.wrede@beyondit.gmbh>

crates/engine/src/cmd/file.rs

@@ -189,7 +189,7 @@ fn write_file(s: &mut Session, path: &str, format: &str, compression: &str, prev
     };
     let bytes = match format {
         "container" => s.design_container(compression, preview, cache)?,
-        "json" => solidcraft_io::write_design(&s.doc),
+        "json" => solidcraft_io::write_design(&s.design()),
         _ => return Err(bad("file.save", "format must be container or json")),
     };
     // Atomic, keeping the version being replaced as `<file>.bak`.
@@ -205,7 +205,7 @@ impl Session {
     pub fn design_container(&self, compression: solidcraft_io::DesignCompression, preview: bool, cache: bool) -> solidcraft_io::Result<Vec<u8>> {
         let geometry = cache.then(|| saved_geometry(self)).flatten();
         let preview = preview.then(|| saved_preview(self)).flatten();
-        solidcraft_io::write_design_container_with_cache(&self.doc, compression, preview.as_ref(), geometry.as_deref())
+        solidcraft_io::write_design_container_with_cache(&self.design(), compression, preview.as_ref(), geometry.as_deref())
     }
 }
 
@@ -289,7 +289,7 @@ fn export_to(s: &Session, path: &str, format: Format, bodies: &[String]) -> Resu
         if !bodies.is_empty() {
             return Err(bad("file.export", "design JSON exports the complete document; omit bodies"));
         }
-        solidcraft_io::write_design(&s.doc)
+        solidcraft_io::write_design(&s.design())
     } else if format == Format::Step && bodies.is_empty() && !s.doc.occurrences.is_empty() {
         solidcraft_io::step_assembly(&s.doc, &s.doc.painted(&s.model.state()), &name)?
     } else {