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/data/local_data/sky/README.md

@@ -8,7 +8,7 @@ whole-globe view and show compression blocks.
 | File | Content | Size |
 | ---- | ------- | ---- |
 | `bright_stars.json` | 9,096 stars: J2000 RA/Dec, V magnitude, B−V | 235 KB |
-| `milkyway_{px,nx,py,ny,pz,nz}.webp` | Milky Way glow, six 1024² cube faces, no stars | 247 KB |
+| `milkyway_{px,nx,py,ny,pz,nz}.webp` | Milky Way glow, six 1024² cube faces, no stars | 85 KB |
 
 ## Stars — Yale Bright Star Catalogue
 
@@ -45,7 +45,8 @@ Centre.
 4096×2048, stars removed with a 7×7 morphological opening, two planets
 (Jupiter, by position) painted over with the median of the surrounding sky,
 blurred (σ = 0.4°), reprojected from galactic coordinates to ICRF cube faces,
-black level set at the darkest 25 % of the sky, gamma-encoded and saved as
+black level set at the median sky with a soft knee (the faint glow away
+from the galactic plane fades to black), gamma-encoded and saved as
 WebP. Build:
 `node scripts/build-milky-way.mjs <eso0932a.tif>`.
 

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);
+});