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.

docs/CURRENT-STATE.md

@@ -2205,7 +2205,7 @@ none is a cold hue, so the brightest marks are the strongest fires; ~90% of
 detections are under 20 MW and stay dim red. Glow sprites and fire-card
 accents read that ramp; confidence and brightness do not affect the color.
 
-At far zoom (camera above ~3,000 km) the layer drapes a Snap Map–style
+At far zoom (camera above ~4,500 km) the layer drapes a Snap Map–style
 heatmap instead of individual markers: the shown detections are linearly binned
 (each detection's weight shared between the four nearest pixel centres, so spots
 sit at the true position and stay round) on a
@@ -2221,14 +2221,15 @@ nothing burns (`src/data/firmsHeatRaster.js`). It is one full-globe imagery
 tile on the active imagery host (`src/layers/firms/heatImagery.js`), rebuilt
 on each data refresh or filter change (the first far-band paint at once,
 later ones 150 ms after the last change). Heatmap and markers are both kept
-built at every zoom and cross-fade by camera height between 2,400 and
-3,600 km, every frame, so a fast zoom never shows a gap: the heatmap fades by
+built at every zoom and cross-fade by camera height between 3,600 and
+5,400 km, every frame, so a fast zoom never shows a gap: the heatmap fades by
 imagery `alpha` (a hidden Cesium imagery layer is unmapped from the globe
 tiles and re-mapped late when shown), the markers by sprite alpha, and the
-marker collection is hidden once the heatmap is whole. The 750–3,000 km
-markers band draws up to 20,000 detections (the strongest in the padded
+marker collection is hidden once the heatmap is whole. The 750–4,500 km
+markers band draws up to 40,000 detections (the strongest in the padded
 view), so the weak fires the heatmap shows stay on screen as markers when it
-fades out. "N FIRES" cards from a 2° / 1° aggregation still label the hottest
+fades out; it reads each marker's already-built anchor instead of the ground
+floor cache, which only the close band warms. "N FIRES" cards from a 2° / 1° aggregation still label the hottest
 areas; their accent is the strongest fire's FRP color.
 
 The Active Fires row carries a "Fire intensity (FRP)" slider on a log scale