Joint dialog: say why picks on one component make no joint (#1367)

The Joint and As-built Joint dialogs explain why picks make no joint and offer Create Components from Bodies when every pick is on one component; joint.create and joint.as_built suggest component.from_bodies. Labels in all catalogs. Fixes #1239.

crates/engine/src/cmd/joints.rs

@@ -318,8 +318,14 @@ fn add_joint(s: &mut Session, j: Joint, p: &Value) -> Result<Value> {
     if s.doc.assembly.joints.len() >= 10_000 {
         return Err(EngineError::Other("too many joints".into()));
     }
+    // Several parts as bodies of one component (a multi-body STEP import, #1239): say how to
+    // get two components.
     if j.a.occurrence == j.b.occurrence {
-        return Err(bad("joint.create", "a joint needs two different occurrences"));
+        return Err(bad(
+            "joint.create",
+            "a joint needs two different occurrences: both origins are on one component; give each part a component of its own first \
+             (component.from_bodies)",
+        ));
     }
     let id = j.id;
     let name = j.name.clone();

crates/engine/src/cmd/joints_tests.rs

@@ -247,6 +247,22 @@ fn faces_mate_by_default() {
     assert!(near(place(&s, "B", Vec3::new(25.0, 5.0, 10.0)), Vec3::new(5.0, 5.0, 0.0)), "{:?}", place(&s, "B", Vec3::new(25.0, 5.0, 10.0)));
 }
 
+/// #1239: two parts as bodies of one component (a multi-body STEP import). The refusal names the
+/// way out, and after Create Components from Bodies the same origins make the joint.
+#[test]
+fn bodies_of_one_component_get_components_first() {
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10}));
+    run(&mut s, "solid.box", json!({"length": 10, "width": 10, "height": 10, "corner": [20, 0, 0]}));
+    let p = json!({"type": "rigid", "a": {"face": [5, 5, 10]}, "b": {"face": [25, 5, 0]}});
+    let e = s.execute("joint.create", &p).map(|_| ()).unwrap_err().to_string();
+    assert!(e.contains("two different occurrences") && e.contains("component.from_bodies"), "{e}");
+    run(&mut s, "component.from_bodies", json!({"bodies": ["Body1", "Body2"]}));
+    run(&mut s, "joint.create", p);
+    // The first joint's A side stays; B lands on top of it.
+    assert!(near(place(&s, "Body2", Vec3::new(25.0, 5.0, 0.0)), Vec3::new(5.0, 5.0, 10.0)));
+}
+
 /// #527: an assembly with its fasteners passes 200 bodies quickly; the clash check must still
 /// cover the whole design. 201 separate 1 mm cubes 2 mm apart, plus one cube pushed half-way
 /// into the first: exactly that pair overlaps, by 0.5 mm³.