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/README.md

@@ -94,7 +94,7 @@ The host-owned catalog classifies all 48 entity variants and tracks every
 traced field, including fields marked `unmapped`. Query it with
 `doc.coverage()` or `doc.coverage("Line")`; each row includes source field,
 type, access, and current validation status. Geometry edits remain limited to
-the 33 mapped kinds.
+the 34 mapped kinds.
 For any entity, `entity.snapshot` returns a detached dictionary of its raw
 CAD fields, including `snapshot["common"]` and kind-specific nested values.
 It can inspect an unmapped kind; editing the dictionary does