face.delete heal: false leaves an open surface body (#861)

Delete Face takes heal (default true, as before). With heal: false the picked faces are removed and the gap stays open, leaving the remaining faces as a surface body (kernel remove_faces); deleting every face is refused. Saved only when false, noted under format 18.

crates/doc/src/eval.rs

@@ -4134,7 +4134,7 @@ fn eval_feature(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st:
             }
             Ok(())
         }
-        FeatureKind::DeleteFace { faces, body } => {
+        FeatureKind::DeleteFace { faces, body, heal } => {
             let i = body_at(st, body, faces)?;
             let Some(mb) = st.bodies.get(i).cloned() else { return Err(DocError::Invalid("body".into())) };
             let mut picked: Vec<usize> = Vec::new();
@@ -4146,7 +4146,8 @@ fn eval_feature(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st:
                 }
             }
             let all = kernel::with_same_surface(&mb.body, &picked);
-            let nb = Traced::step(kernel::delete_faces(&mb.body, &all)?, &[&mb.traced()]);
+            let left = if *heal { kernel::delete_faces(&mb.body, &all)? } else { kernel::remove_faces(&mb.body, &all)? };
+            let nb = Traced::step(left, &[&mb.traced()]);
             if let Some(slot) = st.bodies.get_mut(i) {
                 *slot = ModelBody::traced_new(mb.name, nb, mb.feature);
             }