Keep weak Active Fires as markers through the hand-off

The 750–3,000 km markers band now draws up to 20,000 detections instead
of 2,500. At the 2,400 km end of the cross-fade the padded view over
Africa holds ~21,000 fires, and the old cap kept only those above
~17 MW, so most heat spots faded into empty ground as the heatmap went
out. A rebuild of that many costs ~60 ms, and only in the densest views.

docs/CURRENT-STATE.md

@@ -2225,7 +2225,10 @@ 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
 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. "N FIRES" cards from a 2° / 1° aggregation still label the hottest
+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
+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
 areas; their accent is the strongest fire's FRP color.
 
 The Active Fires row carries a "Fire intensity (FRP)" slider on a log scale