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/tooling/skyAssets.test.mjs

@@ -9,6 +9,7 @@ import {
   cubeFaceDirection,
   directionToPlateCarree,
   icrfToGalactic,
+  smoothstep,
   softBlack,
 } from '../../scripts/build-milky-way.mjs';
 import { raDecToUnitVector } from '../sky/stars.js';
@@ -121,3 +122,11 @@ test('the black level eases in instead of cutting the faint sky into contours',
   assert.ok(softBlack(0.003) < 0.0003);
   close(softBlack(1), 1 - 0.03, 1e-3);
 });
+
+test('the faint-glow blend weight eases between its edges', () => {
+  assert.equal(smoothstep(0.1, 0.2, 0), 0);
+  assert.equal(smoothstep(0.1, 0.2, 0.1), 0);
+  close(smoothstep(0.1, 0.2, 0.15), 0.5, 1e-9);
+  assert.equal(smoothstep(0.1, 0.2, 0.2), 1);
+  assert.equal(smoothstep(0.1, 0.2, 9), 1);
+});