HRRR publishes f21-f48 at 3-hourly intervals in addition to the
hourly f01-f18 we already fetch. This extends the grid_tasks seed
from 19 rows (1 analysis + 18 forecast) to 29 rows (1 analysis +
18 hourly + 10 3-hourly), and bumps the UI forecast window
constant from 18h to 48h so the map timeline and path calculator
forecast chart automatically show the extended horizon.
No Rust logic changes needed — the pipeline is data-driven and
u8 forecast_hour already handles f48.
