testkit: run the meshing skill on the corpus and samples (cargo xtask skill-results)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/plan.rs

@@ -1079,7 +1079,7 @@ fn apply(s: &mut Session, p: &Value) -> Result<Value> {
 /// The faces of `bodies` (as the design has them) that no element face of the mesh is tagged
 /// with: a set, load or relaxation by such a face finds nothing. Only a face narrower than the
 /// elements beside it should be one.
-fn untagged_faces(s: &Session, bodies: &[String]) -> Vec<Value> {
+pub fn untagged_faces(s: &Session, bodies: &[String]) -> Vec<Value> {
     let c = s.model.classification();
     let mut tagged: BTreeSet<u32> = BTreeSet::new();
     for t in c.elements().values() {