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.

src/data/layerState.js

@@ -298,7 +298,7 @@ function satelliteFilterOption() {
 }
 
 const OPTION_GROUPS = Object.freeze({
-  // Active Fires intensity filter and heatmap spot size (HAL-676). `minFrp` is whole megawatts on
+  // Active Fires intensity filter (HAL-676). `minFrp` is whole megawatts on
   // the codec side; the panel's log slider rounds to it. Both default to
   // "show everything": many of the strongest fires are rated low confidence.
   // The bounds and modes mirror MIN_FRP_MW_MAX / FIRMS_CONFIDENCE_MODES in
@@ -311,9 +311,6 @@ const OPTION_GROUPS = Object.freeze({
       nominal: 'n',
       high: 'h',
     }),
-    // Heatmap spot size in percent; mirrors HEAT_SPOT_SIZE_* in
-    // firmsHeatRaster.js.
-    boundedIntegerOption('spotSize', 's', 100, { min: 50, max: 400 }),
   ]),
   traffic: Object.freeze([
     enumOption('roadMode', 'r', null, ['tomtom', 'osm', 'hybrid'], {