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/entity_coverage_policy.json

@@ -377,6 +377,17 @@
       "faces",
       "edges",
       "override_option"
+    ],
+    "Helix": [
+      "layer",
+      "axis_base_point",
+      "start_point",
+      "axis_vector",
+      "radius",
+      "turns",
+      "turn_height",
+      "handedness",
+      "constraint"
     ]
   },
   "readable": {
@@ -432,6 +443,11 @@
     ],
     "Mesh": [
       "version"
+    ],
+    "Helix": [
+      "spline",
+      "major_version",
+      "maintenance_version"
     ]
   },
   "aliases": {