Two related optimizations in the propagation chain hot path. Both
land on the f01..f18 step that was previously OOM-killing prod
pods after the HRRR pressure-level footprint halved wasn't enough
to fit inside 4 Gi.
1. Skip the GridCache broadcast on forecast hours.
/weather only ever renders the analysis hour (latest_grid_valid_time
feeds the map). Building 92k rows, serializing them through PubSub,
and rebuilding the {lat,lon}→row map on all three replicas was
pure waste for f01..f18 — no consumer was reading that data. Only
f00 now calls build_grid_cache_rows + broadcast_put. Point lookups
for non-analysis hours still work through ProfilesFile on disk
(weather_point_detail_from_profiles/3) exactly as before.
2. Fold replace_scores into a single streaming pass.
The old path did `Enum.to_list/1` on the ~460k-entry score stream
followed by `Enum.group_by/2`, holding two full copies of the grid
before any file was written. A single `Enum.reduce/3` that folds
each score into a per-band accumulator keeps only one copy and
eliminates the group_by intermediate entirely. The public
signature — an Enumerable in, {:ok, count} out — is unchanged.
Graham McIntire
32fdb2583d