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]>

DATA_SOURCES.md

@@ -198,6 +198,8 @@ The [Bhote Koshi event pack](public/events/bhote-koshi-2026/README.md), under `p
 | **DataSF Analysis Neighborhoods** (41 SF neighborhood polygons)             | `neighborhoods/`                  | **PDDL 1.0** (public domain)                                                                              | ✅ (no restrictions)                             | "City & County of San Francisco — DataSF" (courtesy — not legally required) |
 | **Natural Earth countries, states and provinces** (260 admin-0/map units + 4,587 admin-1 polygons)             | `natural_earth/`                  | **Public domain**                                                                                         | ✅ (no restrictions)                             | "Made with Natural Earth" (courtesy credit — not legally required)          |
 | **US Census Bureau counties** (3,235 county polygons)                       | `us_census_counties/`             | **Public domain**                                                                                         | ✅ (no restrictions)                             | "U.S. Census Bureau" (courtesy — not legally required)                      |
+| **Yale Bright Star Catalogue** (9,096 stars, sky background)                | `sky/`                            | **Public domain**                                                                                         | ✅ (no restrictions)                             | "Yale Bright Star Catalogue (Hoffleit & Warren, via CDS)" (courtesy credit) |
+| **ESO Milky Way panorama** (eso0932a, blurred cube map, sky background)     | `sky/`                            | **CC BY 4.0**                                                                                             | ✅ (attribution; changes stated)                 | "ESO/S. Brunier"                                                            |
 | **CCTV ground heights** (3,445 cameras)                                     | `cctv_ground_heights/`            | Precomputed camera placement heights, aligned to work with Google Photorealistic 3D Tiles (folder README) | —                                                | —                                                                           |
 | **CCTV Camera Database** model-enrichment extract v2.26.0 (17 camera-model records) | `src/layers/cctv/modelSpecs.js` | **CC0-1.0** (public domain dedication) | ✅ (no restrictions) | "Specs: cctv-database.com" (courtesy — not legally required) |
 
@@ -270,6 +272,20 @@ https://www.naturalearthdata.com/about/terms-of-use/). We credit anyway: "Made w
 Earth". Registration in the in-app `dataCredits.js` attribution list ships with the resolver
 wiring (see below).
 
+### Sky background (`sky/`)
+
+The space behind the globe is drawn from two bundled sources (details, hashes
+and build commands in [`sky/README.md`](src/data/local_data/sky/README.md)):
+
+- **Stars:** the Yale Bright Star Catalogue, 5th Revised Edition (Hoffleit &
+  Warren 1991, CDS catalogue V/50) — public domain; credited as a courtesy.
+- **Milky Way glow:** ESO image eso0932a, "The Milky Way panorama" — **CC BY
+  4.0, credit "ESO/S. Brunier"**. Modified: stars removed, blurred, reprojected
+  to ICRF cube faces and tone-mapped, as the folder README states.
+
+NASA SVS "Deep Star Maps 2020" was not used for the glow: its Milky Way layer
+derives from Gaia DR2, which ESA licenses CC BY-NC 3.0 IGO.
+
 ### DataSF Analysis Neighborhoods (`neighborhoods/`)
 
 `neighborhoods/san-francisco.json` bundles the City & County of San Francisco's official