Cross-fade Active Fires markers and heatmap; fix the spot size

The far-zoom heatmap now uses the ½× grid (4096 × 2048, ≈10 km per
pixel, one-pixel blur), so a lone fire's spot matches its marker at the
3,000 km hand-off. The spot size slider and its share option are gone.

Markers and heatmap are both kept built at every zoom and cross-fade by
camera height between 2,400 and 3,600 km on every frame, so a fast zoom
never shows a beat with neither. The heatmap fades by imagery alpha:
hiding a Cesium imagery layer unmaps it from the globe tiles, and showing
it again re-mapped them late, which was the gap at the band edge. Under
the markers the heatmap is built 150 ms after the data or filter changes.

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

@@ -4,9 +4,9 @@
   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,
-  one distinct spot per fire, empty where nothing burns, with a ½×–4× spot
-  size slider. The filter applies once before rendering, so markers, cards,
-  the heatmap, context and the top-fires query agree; share links keep the
-  filter and the spot size (`w` token: `f`, `c`, `s`), and `get_active_fires`
+  one distinct spot per fire, empty where nothing burns, and cross-fades with
+  the markers around 3,000 km with no gap. 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
-  / 1× (gekh, #1426).
+  (gekh, #1426).