docs(changelog): add the starfield sky entry

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

changelog.d/1243-starfield-sky.md

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+- Replace the blocky JPEG sky around the globe with the 9,096 stars of the
+  Yale Bright Star Catalogue, drawn as sharp points where each star really is
+  at the scene clock, over a soft Milky Way built from ESO/S. Brunier's
+  panorama (CC BY 4.0). The Sun and Moon stay; the new assets weigh 482 KB
+  (gekh, #1243).