feat(fires): far zoom shows a Snap Map-style heatmap instead of heat squares

The global and regional LOD bands drape the heat raster on the active
imagery host and draw no rectangle entities; the 2/1 degree aggregation now
only feeds the "N FIRES" cards, whose accent is the strongest fire's FRP
color. Markers below ~750 km hide the heatmap, as do disable and destroy.
A stale raster is rebuilt immediately on first paint and with a 150 ms
trailing debounce afterwards, so a slider drag does not re-encode on every
step. The shell hands the layer its imagery host, so the heatmap follows
globe and 3D-tiles map stacks.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/layers/firms/heatImagery.js

@@ -55,6 +55,10 @@ export function createHeatImagery({
       raster = next;
       attach();
     },
+    /** True once a raster was handed over (painted or waiting for a host). */
+    hasRaster() {
+      return raster !== null;
+    },
     setShow(value) {
       show = Boolean(value);
       if (layer) layer.show = show;