Add spatial serving and HRRR downloader updates

models/wildfire_fm/README.md

@@ -28,3 +28,11 @@ A machine-readable copy is stored in `input_channels.json`.
 FIRMS detections are used to derive the occupancy target, not as an input
 channel. WFIGS and MTBS are supporting event-level resources and are not part of
 the pretrained occupancy input.
+
+## County and sub-county serving
+
+The checkpoint remains a native-grid model. For web and client migration, use
+`../../spatial_serving/grid_to_polygons.py` to aggregate probability maps to
+county, census tract, block group, fire district, or custom polygon boundaries.
+This supports multiple serving granularities without changing the checkpoint or
+the 16-channel input contract.