Support Color in the Python adapter generator; map Leader.override_color

Color converts as {'kind': ByLayer|None|ByBlock|Index|Rgb, 'value': ...}.
Leader.override_color is now writable; neither DWG nor DXF stores it, which
is recorded as a fourth persistence blocker with a canary assertion.

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

crates/ocs_plugin_api/entity_coverage_policy.json

@@ -281,6 +281,7 @@
       "text_height",
       "text_width",
       "vertices",
+      "override_color",
       "annotation_handle",
       "normal",
       "horizontal_direction",