Move the Active Fires hand-off up to 4,500 km

The heatmap now takes over above 4,500 km instead of 3,000 km, with
the cross-fade between 3,600 and 5,400 km. The markers band caps at
40,000 detections, which covers the ~40,000 in the padded view over
Africa at the low end of the fade, so weak fires still stay as markers.

To keep that rebuild near its old cost, markers outside the close band
read the anchor already built for each fire instead of the ground floor
cache: only the close band warms floors, so the lookup there always
missed. A 40,000-marker rebuild now costs ~55 ms, down from ~120 ms.

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

@@ -5,7 +5,7 @@
   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,
   one distinct spot per fire, empty where nothing burns, and cross-fades with
-  the markers around 3,000 km with no gap, keeping weak fires as markers. The filter applies once before
+  the markers around 4,500 km with no gap, keeping weak fires as markers. 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