fix(engine): pick the face of a named body where bodies meet flush (face_body) (#710)

Extrude and Revolve from a body face take `face_body` to say whose face is meant where faces of several bodies meet flush at the picked point. Without it, an ambiguous pick now names the bodies that meet there instead of saying there is no face. Fixes #703.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/tests.rs

@@ -111,6 +111,22 @@ fn a_body_cut_in_pieces_twice_gets_pieces_of_their_own_names() {
 }
 
 /// A join that bridges two bodies makes them one body, not an overlap of the first and the second.
+#[test]
+fn extrude_a_face_where_another_body_lies_flush_on_it() {
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"length": 20, "width": 20, "height": 4, "body_name": "A"}));
+    run(&mut s, "solid.box", json!({"corner": [10, 0, 4], "length": 10, "width": 20, "height": 1, "body_name": "B"}));
+    // A's top and B's bottom both lie at the point: which one is meant has to be said.
+    let e = s.execute("solid.extrude", &json!({"face": [15, 5, 4], "distance": "2 mm", "operation": "new"})).unwrap_err().to_string();
+    assert!(e.contains("A") && e.contains("B") && e.contains("face_body"), "{e}");
+    let r = run(&mut s, "solid.extrude", json!({"face": [15, 5, 4], "face_body": "A", "distance": "2 mm", "operation": "new", "body_name": "C"}));
+    let m = run(&mut s, "inspect.measure", json!({"bodies": ["C"]}));
+    let v = m["total"]["volume_mm3"].as_f64().unwrap();
+    assert!((v - 20.0 * 20.0 * 2.0).abs() < 1e-6, "{v} {r}");
+    let e = s.execute("solid.extrude", &json!({"face": [15, 5, 4], "face_body": "Nope", "distance": "2 mm"})).unwrap_err().to_string();
+    assert!(e.contains("Nope"), "{e}");
+}
+
 #[test]
 fn join_bridging_two_bodies_merges_them() {
     let mut s = Session::default();