Map Underlay in the Python host model; record DXF rotation blocker

Scripts place an underlay against an existing UnderlayDefinition; the host
checks that the definition exists and matches the underlay type.
staged_python_underlay_lifecycle_over_real_ipc passes every gate except DXF
persistence: acadrust's reader returns the degree-valued rotation
unconverted and repeated saves compound it. Pinned by canary expectations.

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

plugins/opencad-python/entity_manifest.json

@@ -32,7 +32,8 @@
     "Mesh",
     "Helix",
     "RasterImage",
-    "Wipeout"
+    "Wipeout",
+    "Underlay"
   ],
   "manual_kinds": ["Dimension"],
   "base_fields": ["handle", "layer", "owner_handle"],
@@ -360,6 +361,12 @@
         "definition_handle": { "read_only": true },
         "definition_reactor_handle": { "read_only": true }
       }
+    },
+    "Underlay": {
+      "fields": {
+        "definition_handle": { "required": true },
+        "insertion_point": { "required": true }
+      }
     }
   }
 }