fix(fires): weight the far-zoom heatmap by fire radiative power

Heat was a detection count, so raising the minimum-FRP slider removed most
detections and the hot spots faded to blue even where strong fires still
burned. Each detection now adds its FRP in megawatts (0.5 MW floor), so weak
fires carry little heat and filtering them out keeps strong-fire areas hot.
On the live payload, hot spots that stay yellow or hotter at >=100 MW rise
from 7% to 17% and their median heat from 0.10 to 0.38.

docs/CURRENT-STATE.md

@@ -2096,7 +2096,9 @@ not affect the color.
 
 At far zoom (camera above ~3,000 km) the layer drapes a Snap Map–style
 heatmap instead of individual markers: the shown detections are binned on a
-global 2048 × 1024 grid, each weighted `1 + log10(1 + FRP)`, Gaussian-blurred,
+global 2048 × 1024 grid, each weighted by its FRP in megawatts (so heat is
+radiated power, and raising the FRP slider does not dim areas where strong fires
+burn), Gaussian-blurred,
 normalized on a log scale against the 99.5th percentile and mapped from blue
 through cyan, green, yellow and orange to red. Nothing is painted where
 nothing burns (`src/data/firmsHeatRaster.js`). It is one full-globe imagery