Insert Design on the ASSEMBLE panel and in the File menu (#1240) (#1405)

component.insert_design (a saved part as a linked component) had no button, so an
assembly of separately saved parts looked impossible. It is now on the ASSEMBLE panel
of every tab and in the File menu; it asks for the file, and a STEP or mesh file picked
there is inserted as Insert STEP or Insert Mesh would.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/workspace.rs

@@ -11,6 +11,7 @@ const ASSEMBLE: &[&str] = &[
     "component.create",
     "joint.create",
     "joint.as_built",
+    "component.insert_design",
     "joint.origin",
     "joint.rigid_group",
     "joint.drive",
@@ -212,4 +213,36 @@ mod tests {
         assert!(ids("PLASTIC", "MODIFY").contains(&"solid.shell"));
         assert!(panels("PLASTIC").is_some_and(|p| p.first() == Some(&"SETUP")));
     }
+
+    /// #1240: a saved part reaches an assembly from the ASSEMBLE panel (and the File menu) as a
+    /// linked component, and a STEP file picked there is inserted as Insert STEP would.
+    #[test]
+    fn insert_design_is_on_the_assemble_panel_and_inserts_a_saved_part() {
+        use serde_json::json;
+        for tab in ["SOLID", "SHEET METAL", "PLASTIC"] {
+            assert!(ids(tab, "ASSEMBLE").contains(&"component.insert_design"), "{tab}");
+        }
+        assert!(!ids("SOLID", "ASSEMBLE")[..3].contains(&"component.insert_design"), "Component, Joint and As-built Joint keep the big buttons");
+        let mut h = crate::scenario::Harness::new();
+        let part = h.dir.join("part1.solidcraft");
+        let mut s = solidcraft_engine::Session::default();
+        s.execute("solid.box", &json!({"length": 10, "width": 20, "height": 30})).unwrap();
+        s.execute("file.save", &json!({"path": part})).unwrap();
+        s.execute("file.export", &json!({"path": h.dir.join("part2.step")})).unwrap();
+        h.step(&json!({"choose_file": "part1.solidcraft"})).unwrap();
+        h.step(&json!({"start": "component.insert_design"})).unwrap();
+        h.frames(3);
+        let links = h.app.session.execute("component.links", &json!({})).unwrap();
+        assert_eq!(links["links"][0]["name"], "part1", "{links}");
+        assert_eq!(h.app.session.world_state().bodies.len(), 1);
+        h.step(&json!({"choose_file": "part2.step"})).unwrap();
+        h.step(&json!({"start": "component.insert_design"})).unwrap();
+        for _ in 0..600 {
+            if h.app.session.world_state().bodies.len() == 2 {
+                break;
+            }
+            h.frames(1);
+        }
+        assert_eq!(h.app.session.world_state().bodies.len(), 2, "the STEP file's body is inserted");
+    }
 }