Brighten the heatmap edge so weak fire regions stay visible

The heat scale is set by the hottest regions, so most of the world sits
at the low end. The ember edge was a dark red at 35% opacity and nearly
vanished into forest and savanna imagery: over the Americas the spots
were there but hard to see. The edge is now a bright red starting at
58% opacity, which reads at least as well as the old blue did.

docs/CURRENT-STATE.md

@@ -2189,8 +2189,9 @@ global 2048 × 1024 grid, each weighted by its FRP in megawatts (so heat is
 radiated power, and raising the FRP slider does not dim areas where strong fires
 burn), Gaussian-blurred,
 normalized on a log scale against the 99.5th percentile and mapped through the
-same colors, from a faint ember edge through red, orange and gold to a pale
-white-hot core. Nothing is painted where
+same hues, from a bright red edge through orange and gold to a pale
+white-hot core. The edge starts more than half opaque, so regions far below
+the hottest ones still read over forest and savanna imagery. Nothing is painted where
 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 paint at once, later ones

src/data/firmsHeatRaster.js

@@ -9,17 +9,20 @@
 export const HEAT_FLOOR = 0.03;
 
 /**
- * The FRP marker ramp as a continuous glow: faint ember at the edges, a
- * pale white-hot core where the most power burns.
+ * The FRP marker ramp as a continuous glow, from a red edge through orange
+ * and gold to a pale white-hot core. The edge is a bright red, not the
+ * markers' dark ember, and starts more than half opaque: most of the world
+ * sits low on a scale set by the hottest regions, and a dark translucent red
+ * vanishes into forest and savanna imagery.
  */
 export const HEAT_PALETTE = Object.freeze([
-  { at: 0.0, rgb: [158, 42, 27], alpha: 0 },
-  { at: 0.08, rgb: [158, 42, 27], alpha: 0.35 },
-  { at: 0.25, rgb: [210, 64, 31], alpha: 0.55 },
-  { at: 0.45, rgb: [240, 106, 40], alpha: 0.65 },
-  { at: 0.65, rgb: [251, 154, 46], alpha: 0.75 },
-  { at: 0.82, rgb: [253, 200, 74], alpha: 0.82 },
-  { at: 1.0, rgb: [255, 243, 176], alpha: 0.9 },
+  { at: 0.0, rgb: [232, 58, 30], alpha: 0 },
+  { at: 0.08, rgb: [232, 58, 30], alpha: 0.58 },
+  { at: 0.25, rgb: [242, 86, 34], alpha: 0.7 },
+  { at: 0.45, rgb: [248, 122, 40], alpha: 0.76 },
+  { at: 0.65, rgb: [252, 166, 52], alpha: 0.82 },
+  { at: 0.82, rgb: [254, 208, 92], alpha: 0.87 },
+  { at: 1.0, rgb: [255, 245, 190], alpha: 0.92 },
 ]);
 
 export function heatColorAt(t) {

src/data/firmsHeatRaster.test.mjs

@@ -18,7 +18,8 @@ test('palette runs ember to white-hot and starts fully transparent', () => {
   assert.equal(heatColorAt(0.01)[3], 0, 'below the floor nothing is painted');
   const low = heatColorAt(0.15);
   const high = heatColorAt(1);
-  assert.ok(low[0] > low[1] && low[1] > low[2], 'low heat is an ember red');
+  assert.ok(low[0] > low[1] && low[1] > low[2], 'low heat is red');
+  assert.ok(low[3] >= 0.5 * 255, 'low heat stays visible on any imagery');
   assert.ok(high[1] > low[1] && high[2] > low[2], 'the core is paler');
   assert.ok(high[3] > low[3], 'hotter is more opaque');
   for (let i = 1; i <= 100; i++) {