fix(sky): ease the Milky Way black level in at the median sky

Away from the galactic plane the ESO panorama keeps a mottled glow of
unresolved stars and dust. With the black level at the darkest quarter of
the sky and a hard cut, it showed as grey clouds with black holes around
the globe. The black level now sits at the median sky and eases in with a
soft knee on luminance, keeping each pixel's hue: high galactic latitudes
are black, the band keeps its glow. Faces shrink from 247 to 85 KB.

The changelog fragment also notes the anime fade and the new asset weight.

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

src/tooling/skyAssets.test.mjs

@@ -9,6 +9,7 @@ import {
   cubeFaceDirection,
   directionToPlateCarree,
   icrfToGalactic,
+  softBlack,
 } from '../../scripts/build-milky-way.mjs';
 import { raDecToUnitVector } from '../sky/stars.js';
 
@@ -105,3 +106,18 @@ test('the galactic rotation puts the Galactic Centre and pole where the IAU does
   close(u, 0.5 + (360 - 280.47) / 360, 2e-3);
   close(v, 0.5 + 32.89 / 180, 2e-3);
 });
+
+test('the black level eases in instead of cutting the faint sky into contours', () => {
+  assert.equal(softBlack(-0.2), 0);
+  assert.equal(softBlack(0), 0);
+  // Continuous, rising, with no step at the black level.
+  let previous = 0;
+  for (let x = 0.001; x <= 1; x += 0.001) {
+    const y = softBlack(x);
+    assert.ok(y > previous && y - previous < 0.0011, `step at ${x.toFixed(3)}`);
+    previous = y;
+  }
+  // Starts flat, then joins the straight line just below it.
+  assert.ok(softBlack(0.003) < 0.0003);
+  close(softBlack(1), 1 - 0.03, 1e-3);
+});