fix(sky): blur the faint Milky Way glow wider so unresolved stars leave no clumps

The fine 0.4 degree blur kept the residual glow of unresolved stars as
mottled patches at the band's edges. Blend towards a 2 degree blur where
the coarse copy is faint (smoothstep over 0.5-8 % of the range), so the
core keeps its dust lanes and the faint sky goes smooth. Faces 85 -> 62 kB.

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 | 85 KB |
+| `milkyway_{px,nx,py,ny,pz,nz}.webp` | Milky Way glow, six 1024² cube faces, no stars | 62 KB |
 
 ## Stars — Yale Bright Star Catalogue
 
@@ -44,7 +44,8 @@ Centre.
 **Changes made** (CC BY 4.0 asks for them to be stated): resampled to
 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,
+blurred (σ = 0.4°, blending to σ = 2° in the faint glow so unresolved
+stars leave no clumps), reprojected from galactic coordinates to ICRF cube faces,
 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: