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/data/dataCredits.js

@@ -386,6 +386,13 @@ export const DATA_CREDITS = [
       'ECMWF does not accept any liability whatsoever for any error or omission in the data, their availability, or for any loss or damage arising from their use.',
   },
   // ── Bundled snapshots ───────────────────────────────────────────
+  {
+    key: 'sky-background',
+    html:
+      'Stars: <a href="https://cdsarc.cds.unistra.fr/viz-bin/cat/V/50" target="_blank" rel="noopener">Yale Bright Star Catalogue</a> ' +
+      '(Hoffleit &amp; Warren, via CDS). Milky Way: <a href="https://www.eso.org/public/images/eso0932a/" target="_blank" rel="noopener">ESO/S. Brunier</a> ' +
+      '(<a href="https://creativecommons.org/licenses/by/4.0/" target="_blank" rel="noopener">CC BY 4.0</a>; stars removed, blurred and reprojected)',
+  },
   {
     key: 'dams',
     html: 'Dams: Open Infrastructure Map',