fix(sky): keep the Milky Way out of model lighting, test the sky wiring

Scene reads skyBox._cubeMap as the environment map that lights models, in
the TEME, flipY layout of Cesium's own sky box; the ICRF Milky Way would
light them rotated. Rename the field so models keep Cesium's default.

Test the anime fade on a real StarrySky and that the viewer gets the sky
as an option (skyBox: false would drop the Sun and Moon). Correct three
comments on the star limit and the Milky Way levels.

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

src/sky/starrySky.js

@@ -180,7 +180,10 @@ export class StarrySky {
     this._loading = false;
     this._starData = undefined;
     this._faceImages = undefined;
-    this._cubeMap = undefined;
+    // Not `_cubeMap`: Scene reads `skyBox._cubeMap` as the environment map
+    // that lights models, in the TEME, flipY layout of its own sky box. This
+    // ICRF map would light them rotated, so models keep Cesium's default.
+    this._milkyWayCubeMap = undefined;
     this._milkyWayCommand = undefined;
     this._milkyWayAttributes = undefined;
     this._starCommand = undefined;
@@ -191,7 +194,7 @@ export class StarrySky {
     };
     this._milkyWayUniforms = {
       ...uniforms,
-      u_milkyWay: () => this._cubeMap,
+      u_milkyWay: () => this._milkyWayCubeMap,
       u_intensity: () => this.milkyWayIntensity * this.milkyWayVisibility,
     };
     this._starUniforms = {
@@ -301,7 +304,7 @@ export class StarrySky {
     cubeMap.sampler = new Cesium.Sampler({
       minificationFilter: Cesium.TextureMinificationFilter.LINEAR_MIPMAP_LINEAR,
     });
-    this._cubeMap = cubeMap;
+    this._milkyWayCubeMap = cubeMap;
     this._faceImages = undefined;
     const geometry = Cesium.BoxGeometry.createGeometry(
       Cesium.BoxGeometry.fromDimensions({
@@ -440,7 +443,7 @@ export class StarrySky {
       command.vertexArray?.destroy();
       command.shaderProgram?.destroy();
     }
-    this._cubeMap = this._cubeMap?.destroy();
+    this._milkyWayCubeMap = this._milkyWayCubeMap?.destroy();
     this._milkyWayCommand = undefined;
     this._starCommand = undefined;
     this._commands.length = 0;

src/sky/starrySky.test.mjs

@@ -127,3 +127,11 @@ test('the Milky Way glow scales with its visibility, for styles that hide it', (
   assert.equal(sky._milkyWayUniforms.u_intensity(), 0);
   sky.destroy();
 });
+
+test('the Milky Way stays out of the environment map that lights models', () => {
+  // Scene#render hands `skyBox._cubeMap` to image-based lighting, which
+  // expects Cesium's own TEME, flipY layout; this ICRF map would be rotated.
+  const sky = new StarrySky({ requestRender: () => {} });
+  assert.equal('_cubeMap' in sky, false);
+  sky.destroy();
+});

src/sky/stars.js

@@ -13,7 +13,7 @@
  * @module sky/stars
  */
 
-/** Faintest catalogue magnitude; also the reference for brightness. */
+/** Faintest magnitude drawn distinctly (fainter catalogue stars look like it); the brightness reference. */
 export const STAR_LIMIT_MAGNITUDE = 6.5;
 /** Gaussian σ of the faintest star, CSS pixels. */
 export const STAR_MIN_SIGMA_PX = 0.55;