Return a GeoJSON FeatureCollection of per-plant airshed polygons.
Polygons come from the InMAP ISRM v1.2.1 extraction pipeline
(scripts/build_airshed_polygons.py); each feature covers the
cells that receive ~80 % of the plant’s population-weighted
PM2.5 intake under 2005 meteorology. Only plants actually
displaced by this project are included.
Plants not present in the polygon set (non-CONUS, nuclear,
too-new) are silently omitted — the frontend should fall back
to a 500 km circular buffer for those.
See product/plans/phase3-multi-scale-map-ui.md §2.2.