Mirror ScalarFile's 5°×5° chunk layout inside GridCache. Each ETS entry
is now `%{{lat_band, lon_band} => %{{lat, lon} => row}}` instead of a
flat `%{{lat,lon} => row}`. fetch_bounds/2 walks only the chunks that
intersect the requested viewport, so latest-hour pans become
proportional to visible chunks (~2-4) rather than O(92k cells).
fetch/1, fetch_point/3, put/2 and broadcast_put/2 keep their
pre-existing contracts.
Also closes the open items in docs/weather.md: Phase 3 is now done end
to end. The only remaining future-work item is moving WeatherLayers.derive
itself into the Rust pipeline so the BEAM doesn't pay the one-time
materialization pass per new valid_time — small runtime savings vs.
significant port effort, so left as documented future work.
Graham McIntire
1d72f52dad