fix(fires): paint aggregated heat cells warm by density, not by strongest FRP

Coloring each far-zoom cell with the ramp stop of its strongest detection
turned a burning region into a blue / teal / lime mosaic: neighbouring
cells flipped hue on one outlier and the cold stops did not read as fire.
Cells say where it burns, so they go back to the heat-normalized score
(density, night share, max FRP) on the warm end of the ramp — lime, amber,
red — with the cell card accent matching. Single detections keep the full
six-stop FRP ramp.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

changelog.d/1426-fire-intensity-filter.md

@@ -2,7 +2,8 @@
   cold → hot ramp (blue · teal · lime · amber · orange · red at 0 · 5 · 20 ·
   50 · 100 · 300 MW), and add a log-scale minimum-FRP slider whose track is the
   ramp, an All / Nominal+ / High confidence switch and an "N of M shown"
-  counter to the layer row. The filter applies once before rendering, so
+  counter to the layer row. Far-zoom heat cells keep a warm density fill
+  (lime · amber · red). The filter applies once before rendering, so
   markers, cards, heat cells, context and the top-fires query agree; share
   links keep it (`w` token: `f`, `c`), and `get_active_fires` takes matching
   `min_frp_mw` and `confidence` arguments. Default 0 MW / All (gekh, #1426).