weather_grid_hrdps_at/2 is the only production caller and backs every
/weather/cells?source=hrdps request, but it read the legacy chunked
.mp.gz dirs exclusively. The pipeline writes <iso>.hrdps.sgrid, so the
snap found a valid_time via list_valid_times_hrdps/0 (which does see
.sgrid) and then read zero chunks for it.
Delegate to read_bounds_hrdps/2, which already does the .sgrid-first,
chunk-fallback read.
Every existing test for this function hand-wrote legacy chunk dirs,
which is how the .sgrid migration slipped past. Added coverage against
a real .sgrid artifact, including the mid-migration case where both
formats exist for one valid_time.
Graham McIntire
ad600361e0