DEM validation mode
===================

After clicking Topography, inspect:
  backend/cache/<HUC>_<hash>/verification.txt
  backend/cache/<HUC>_<hash>/dem_validation.json
  backend/cache/<HUC>_<hash>/manifest.json

The important line is:
  Is this a real numeric DEM? True

The validation checks whether raw_dem.tif and clipped_dem.tif are one-band numeric GeoTIFFs,
not RGB pictures. The TIFF stores elevation in meters; the map/table/legend display feet.
