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>

crates/ocs_plugin_api/build.rs

@@ -790,6 +790,7 @@ fn generate_entity_coverage(out_dir: &Path, registry: &TypeRegistry) {
                     | ("PolygonMesh", _)
                     | ("PolyfaceMesh", _)
                     | ("Mesh", _)
+                    | ("Helix", _)
                     | (
                         "MLine",
                         "flags"