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]>

changelog.d/1243-starfield-sky.md

@@ -1,5 +1,5 @@
 - Replace the blocky JPEG sky around the globe with the 9,096 stars of the
-  Yale Bright Star Catalogue, drawn as sharp points where each star really is
-  at the scene clock, over a soft Milky Way built from ESO/S. Brunier's
-  panorama (CC BY 4.0). The Sun and Moon stay; the new assets weigh 482 KB
-  (gekh, #1243).
+  Yale Bright Star Catalogue, drawn as sharp points at their catalogue
+  positions for the scene clock, over a soft Milky Way built from ESO/S.
+  Brunier's panorama (CC BY 4.0). The Sun and Moon stay, the anime style fades
+  the Milky Way out, and the new assets weigh 319 kB (gekh, #1243).