fix(anime): fade the Milky Way out instead of masking the sky by depth

The far-plane mask in the anime shader read the post-process depth, which
cannot tell the sky from far geometry: with a layer that clamps the near
plane to 0.1 m (satellites) the whole Earth lost its cel bands, keyless
terrain above the ellipsoid horizon did too, and the atmosphere, Sun and
Moon stopped banding.

Restore the anime shader from main and fade the Milky Way glow with the
style instead: VisualEffects sets skyBox.milkyWayVisibility to
1 - anime intensity. scripts/qa-anime-space-mask.mjs checks all three
cases headless and that the anime sky shows no bands.

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

src/sky/starrySky.js

@@ -167,6 +167,8 @@ export class StarrySky {
     /** Matches `SkyBox#show`. */
     this.show = true;
     this.milkyWayIntensity = MILKY_WAY_INTENSITY;
+    /** 0..1 multiplier a style sets to hide the glow (see visualEffects.js). */
+    this.milkyWayVisibility = 1;
     this.starIntensity = STAR_INTENSITY;
     this._requestRender = requestRender;
     this._rotation = Cesium.Matrix3.clone(Cesium.Matrix3.IDENTITY);
@@ -190,7 +192,7 @@ export class StarrySky {
     this._milkyWayUniforms = {
       ...uniforms,
       u_milkyWay: () => this._cubeMap,
-      u_intensity: () => this.milkyWayIntensity,
+      u_intensity: () => this.milkyWayIntensity * this.milkyWayVisibility,
     };
     this._starUniforms = {
       ...uniforms,

src/sky/starrySky.test.mjs

@@ -2,6 +2,7 @@ import test from 'node:test';
 import assert from 'node:assert/strict';
 import * as Cesium from 'cesium';
 import {
+  MILKY_WAY_INTENSITY,
   STARRY_SKY_CESIUM_VERSION,
   SKY_ROTATION_EPSILON_SECONDS,
   StarrySky,
@@ -9,6 +10,9 @@ import {
   isSkyRotationCurrent,
 } from './starrySky.js';
 
+const close = (actual, expected, epsilon = 1e-9) =>
+  assert.ok(Math.abs(actual - expected) <= epsilon, `${actual} vs ${expected}`);
+
 test('the sky-box adapter was validated against this engine version', () => {
   assert.equal(
     Cesium.VERSION,
@@ -115,3 +119,11 @@ for (const iso of ['1970-01-01T00:00:00Z', '2060-01-01T00:00:00Z']) {
     }
   });
 }
+
+test('the Milky Way glow scales with its visibility, for styles that hide it', () => {
+  const sky = new StarrySky({ requestRender: () => {} });
+  close(sky._milkyWayUniforms.u_intensity(), MILKY_WAY_INTENSITY);
+  sky.milkyWayVisibility = 0;
+  assert.equal(sky._milkyWayUniforms.u_intensity(), 0);
+  sky.destroy();
+});