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]>

scripts/package-boundaries.json

@@ -22,7 +22,14 @@
   },
   "viewer": {
     "exports": ["./application/viewer"],
-    "modules": ["src/app/viewer.js", "src/app/atmosphereCompat.js"],
+    "modules": [
+      "src/app/viewer.js",
+      "src/app/atmosphereCompat.js",
+      "src/sky/starrySky.js",
+      "src/sky/stars.js",
+      "src/data/bundledJson.js",
+      "src/renderGovernor.js"
+    ],
     "external": ["cesium"]
   },
   "vite-build": {
@@ -1708,6 +1715,8 @@
       "src/app/surfaceServices.js",
       "src/app/tools.js",
       "src/app/viewer.js",
+      "src/sky/starrySky.js",
+      "src/sky/stars.js",
       "src/app/atmosphereCompat.js",
       "src/bloom.js",
       "src/camera.js",