Feature request: add choropleth support to GeoAxes #622
Description
UltraPlot has geographic plotting support, but it does not currently provide a first-class choropleth API. It would be useful to add GeoAxes.choropleth for polygon geometries and country-keyed inputs so scientists can plot country-level values directly without manually resolving Natural Earth shapes or calling add_geometries themselves. The feature should support the normal UltraPlot colormap and colorbar pipeline, missing-value styling, and examples and tests in the docs.