Map Helix in the Python host model; record DXF handedness blocker

The host rebuilds the derived spline from the generating parameters on
scripted create/edit. staged_python_helix_lifecycle_over_real_ipc passes
every gate except DXF handedness, which acadrust's reader never applies;
the gap is pinned by a canary via per-format expectations in the shared
mesh lifecycle driver.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plugins/opencad-python/src/ocs_module.rs

@@ -4756,7 +4756,7 @@ mod tests {
                 );
             }
         }
-        assert_eq!(emitted.len(), 30);
+        assert_eq!(emitted.len(), 31);
     }
 
     #[cfg(feature = "experimental-host-model")]