geometry: a feature that fails to rebuild names the bodies it changes, and opening warns

The design file stores no measure to compare with, so the rebuild's own
failures are the guard: each failed feature lists the bodies it names (all of
them when it names none), and geometry.open replies with a warning per failure,
which the window shows with the open.

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

crates/engine/src/lib.rs

@@ -734,6 +734,7 @@ mod tests {
         let opened = s.execute("geometry.open", &json!({"path": path})).unwrap();
         assert_eq!(opened["bodies"], json!(["Body1"]));
         assert_eq!(opened["errors"], json!([]));
+        assert_eq!(opened["warnings"], json!([]));
         assert_eq!(s.journal.len(), 1);
 
         let bodies = s.execute("geometry.bodies", &json!({})).unwrap();