Draw each far-zoom fire as a marker-sized heat spot

The heat raster was 2048 px wide with a 3 px blur, so every fire spread
over ~200 km and the far-zoom map read as clouds next to the markers.
Double the grid to ~10 km per pixel and blur by one pixel: a lone fire
is now a spot about as wide as its marker at the 3,000 km hand-off.

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

@@ -4,6 +4,6 @@
   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. Far
   zoom shows a Snap Map–style heatmap in the same colors instead of squares,
-  empty where nothing burns. The filter applies once before rendering, so markers, cards, the heatmap, context and the top-fires query agree; share
+  one marker-sized spot per fire, empty where nothing burns. The filter applies once before rendering, so markers, cards, the heatmap, 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).