feat(sky): replace the blocky default sky box with catalogue stars

Cesium's default Tycho-2 sky box is six 1024² JPEGs stretched 3–6× around
the whole-globe view, so space read as an over-compressed JPEG (HAL-627).

The scene now draws its own sky in the Scene#skyBox slot: the 9,096 stars
of the Yale Bright Star Catalogue as point sprites sized in CSS pixels
(magnitude → width and opacity, B−V → colour), over a soft, star-free
Milky Way cube map built from ESO/S. Brunier's panorama (CC BY 4.0). Both
layers share one ICRF→Earth-fixed rotation at the scene clock, with the
TEME rotation filling in until Cesium's IAU 2006 tables load.

Assets total 482 KB, built by scripts/build-bright-stars.mjs and
scripts/build-milky-way.mjs. Frame time and the idle render governor are
unchanged.

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

src/app/viewer.js

@@ -1,5 +1,6 @@
 import * as Cesium from 'cesium';
 import { applyModelAtmosphereWorkaround } from './atmosphereCompat.js';
+import { StarrySky } from '../sky/starrySky.js';
 
 const PINCH_ZOOM_MULTIPLIER = 8;
 const MAX_PINCH_PIXEL_DELTA = 120;
@@ -119,6 +120,9 @@ export function createApplicationViewer({ container, creditContainer }) {
     selectionIndicator: false,
     infoBox: false,
     baseLayer: false,
+    // Replaced below: the default Tycho-2 cube map is too coarse for the
+    // whole-globe view and its JPEG blocks show around the Earth.
+    skyBox: false,
     creditContainer,
     msaaSamples: 4,
     contextOptions: { webgl: { preserveDrawingBuffer: true } },
@@ -130,6 +134,7 @@ export function createApplicationViewer({ container, creditContainer }) {
     // render loop. See app/atmosphereCompat.js.
     applyModelAtmosphereWorkaround(viewer.scene);
     viewer.scene.globe.show = false;
+    viewer.scene.skyBox = new StarrySky();
     viewer.scene.skyAtmosphere.show = true;
     viewer.scene.skyAtmosphere.atmosphereLightIntensity = 18;
     viewer.scene.skyAtmosphere.saturationShift = -0.12;