fix: point cadkernel at upstream after #1352; unify duplicate-constraint message

- #1352 pinned cadkernel/cadkernel-constraints to ramox81's fork. Its one
  extra commit (cadkernel #67, SymmetricLineDirections) is now on
  HakanSeven12/cadkernel main as the same SHA, so the manifests and lock
  point back at the upstream repo at rev 0493b59.
- SYCONSTRAINT reports a repeated relation with the same message Fix and
  Vertical use, as an error instead of plain output.
- The knot-less spline test consumes the add_entity results.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/app/command_driver.rs

@@ -3591,7 +3591,7 @@ impl OpenCADStudio {
                     self.tabs[i].active_cmd = None;
                     self.tabs[i].snap_result = None;
                     self.command_line
-                        .push_output("The Symmetric constraint already exists.");
+                        .push_error("The constraint already exists on the selected objects.");
                     return Task::none();
                 }
                 let constraints_before = self.tabs[i]