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/app/application.test.mjs

@@ -219,6 +219,17 @@ test('the separate viewer export imports without constructing a browser viewer',
   assert.throws(() => createApplicationViewer({}), /containers are required/);
 });
 
+test('the viewer gets the starry sky as an option, which keeps the Sun and Moon', async () => {
+  const { applicationViewerOptions } = await import('./viewer.js');
+  const { StarrySky } = await import('../sky/starrySky.js');
+  const creditContainer = {};
+  const options = applicationViewerOptions(creditContainer);
+  // CesiumWidget skips scene.sun and scene.moon when skyBox is false.
+  assert.ok(options.skyBox instanceof StarrySky);
+  assert.equal(options.creditContainer, creditContainer);
+  options.skyBox.destroy();
+});
+
 function pinchFixture({ zoomEventTypes } = {}) {
   const handlers = new Set();
   const relayed = [];

src/app/viewer.js

@@ -103,11 +103,9 @@ export function installTrackpadPinchZoom(
   };
 }
 
-/** Create the standard globe viewer in caller-owned, visible containers. */
-export function createApplicationViewer({ container, creditContainer }) {
-  if (!container || !creditContainer)
-    throw new TypeError('Viewer and credit containers are required');
-  const viewer = new Cesium.Viewer(container, {
+/** Cesium.Viewer options of the standard globe viewer. */
+export function applicationViewerOptions(creditContainer) {
+  return {
     timeline: false,
     animation: false,
     baseLayerPicker: false,
@@ -127,7 +125,17 @@ export function createApplicationViewer({ container, creditContainer }) {
     creditContainer,
     msaaSamples: 4,
     contextOptions: { webgl: { preserveDrawingBuffer: true } },
-  });
+  };
+}
+
+/** Create the standard globe viewer in caller-owned, visible containers. */
+export function createApplicationViewer({ container, creditContainer }) {
+  if (!container || !creditContainer)
+    throw new TypeError('Viewer and credit containers are required');
+  const viewer = new Cesium.Viewer(
+    container,
+    applicationViewerOptions(creditContainer),
+  );
   try {
     viewer.targetFrameRate = 60;
     // Before any tile builds a draw command: Cesium's per-vertex model