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/src/entity_coverage.rs

@@ -1684,6 +1684,7 @@ mod tests {
                 "Leader.text_width".to_owned(),
                 "Leader.vertices".to_owned(),
                 "Leader.annotation_handle".to_owned(),
+                "Leader.override_color".to_owned(),
                 "Leader.normal".to_owned(),
                 "Leader.horizontal_direction".to_owned(),
                 "Leader.block_offset".to_owned(),