Apply the same overlay refactor we did for /weather to the propagation
map. Score data used to flow as JSON via LiveView push_event
(update_scores + the 18-hour preload_forecast batch); switching bands
or scrubbing time meant a synchronous server roundtrip + ~MB JSON push
per change.
Now:
1. New /scores/cells endpoint returns a "PSCR" binary cell-pack —
columnar f32 lats/lons + u8 scores, ~3-4× smaller than JSON and
parsed via DataView + typed-array views with no JSON.parse.
2. JS hook owns score fetching. mount/moveend/band/time changes
trigger HTTP fetches against /scores/cells. After the initial
paint, every other forecast hour for the current band is fetched
in parallel in the background — timeline scrubs after the prefetch
finishes are pure cache hits with no network at all.
3. LiveView no longer pushes scores. select_band emits update_band_info
with band_mhz so the client knows what to refetch; select_time and
set_selected_time only update server-side state (URL, point detail
bbox). map_bounds, propagation_updated, and advance_now_cursor all
skip the score push — pack_scores, schedule_bounds_update,
schedule_preload_forecast, the :flush_bounds and :preload_forecast
handle_info clauses, and the start_async :initial_scores lifecycle
(incl. retry_initial_scores event) are all deleted.
Net result: instant band toggles after the first paint, no LiveView
WebSocket payloads larger than the timeline metadata, and ~370 fewer
lines in MapLive.
