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/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