engine: mesh.export.stl writes the outer surface or a set's faces

Binary by default, ASCII on request; in the Mesh tab's Verify panel.

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

crates/model/src/contact.rs

@@ -403,9 +403,20 @@ mod tests {
         assert_eq!(m.skin_faces().len(), 10, "two hex8 sharing a face");
 
         let mut t = Model::default();
-        for (i, p) in [[0., 0., 0.], [1., 0., 0.], [0., 1., 0.], [0., 0., 1.], [0.5, 0., 0.], [0.5, 0.5, 0.], [0., 0.5, 0.], [0., 0., 0.5], [0.5, 0., 0.5], [0., 0.5, 0.5]]
-            .iter()
-            .enumerate()
+        for (i, p) in [
+            [0., 0., 0.],
+            [1., 0., 0.],
+            [0., 1., 0.],
+            [0., 0., 1.],
+            [0.5, 0., 0.],
+            [0.5, 0.5, 0.],
+            [0., 0.5, 0.],
+            [0., 0., 0.5],
+            [0.5, 0., 0.5],
+            [0., 0.5, 0.5],
+        ]
+        .iter()
+        .enumerate()
         {
             t.set_node(i as u32 + 1, *p).unwrap();
         }