point_forecast used a strict \`>= now\` cutoff, so the leftmost
"now" sample was always dropped — the HRRR publishing lag means the
newest analysis file is typically 30–60 min behind the wall clock,
which made the filter evict the very hour the chart most wanted to
anchor on. If the chain ever fell behind by even one step the whole
forecast went empty and the chart disappeared.
Use the same window as available_valid_times: keep everything from
one hour before now onward, and fall back to the single newest file
when nothing is fresh enough. Both the cache-path and store-path
flows go through a shared forecast_window helper so the behavior
stays in lockstep with the map timeline.
Graham McIntire
e227978f2a