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/build-bright-stars.mjs

@@ -0,0 +1,158 @@
+#!/usr/bin/env node
+/**
+ * Build src/data/local_data/sky/bright_stars.json from the Yale Bright Star
+ * Catalogue, 5th Revised Edition (Hoffleit & Warren 1991).
+ *
+ * Source:   CDS VizieR catalogue V/50, https://cdsarc.cds.unistra.fr/viz-bin/cat/V/50
+ * Fetched:  https://cdsarc.cds.unistra.fr/ftp/V/50/catalog.gz
+ * Format:   fixed-width ASCII, 197 bytes per record, byte layout in
+ *           https://cdsarc.cds.unistra.fr/ftp/V/50/ReadMe
+ * License:  public domain (Yale University Observatory / NASA ADC); CDS asks
+ *           for an acknowledgement, which DATA_SOURCES.md carries.
+ *
+ * Transform (deterministic):
+ *   1. J2000 RA/Dec (bytes 76-90), V magnitude (103-107) and B−V (110-114).
+ *   2. The 14 records without a J2000 position (novae and other objects later
+ *      found not to be stars) are dropped.
+ *   3. RA/Dec rounded to 3 decimals of a degree (3.6″ — a fraction of a pixel
+ *      at any field of view the globe uses), magnitudes to 2.
+ *   4. Missing B−V is stored as null; the renderer gives it a neutral colour.
+ *   5. Records keep catalogue (HR) order for a stable diff.
+ *
+ * Usage:
+ *   node scripts/build-bright-stars.mjs [catalog | catalog.gz]
+ * With no argument it downloads the catalogue from CDS.
+ */
+
+import fs from 'node:fs';
+import path from 'node:path';
+import zlib from 'node:zlib';
+import { fileURLToPath } from 'node:url';
+
+const SOURCE_URL = 'https://cdsarc.cds.unistra.fr/ftp/V/50/catalog.gz';
+const OUT = path.join(
+  path.dirname(fileURLToPath(import.meta.url)),
+  '..',
+  'src',
+  'data',
+  'local_data',
+  'sky',
+  'bright_stars.json',
+);
+
+/** Slice 1-based inclusive byte columns the way the CDS ReadMe numbers them. */
+function columns(line, from, to) {
+  return line.slice(from - 1, to).trim();
+}
+
+/** Parse a fixed-width float; blank fields are null. */
+function parseField(text) {
+  if (text === '') return null;
+  const value = Number(text);
+  return Number.isFinite(value) ? value : null;
+}
+
+/**
+ * Parse one BSC5 record. Returns null for records without a J2000 position
+ * or V magnitude.
+ */
+export function parseBscRecord(line) {
+  const hr = parseField(columns(line, 1, 4));
+  const raH = parseField(columns(line, 76, 77));
+  const raM = parseField(columns(line, 78, 79));
+  const raS = parseField(columns(line, 80, 83));
+  const sign = columns(line, 84, 84);
+  const deD = parseField(columns(line, 85, 86));
+  const deM = parseField(columns(line, 87, 88));
+  const deS = parseField(columns(line, 89, 90));
+  const vmag = parseField(columns(line, 103, 107));
+  const bv = parseField(columns(line, 110, 114));
+  if (
+    hr === null ||
+    raH === null ||
+    raM === null ||
+    raS === null ||
+    deD === null ||
+    deM === null ||
+    deS === null ||
+    vmag === null ||
+    (sign !== '+' && sign !== '-')
+  )
+    return null;
+  const raDeg = (raH + raM / 60 + raS / 3600) * 15;
+  const decDeg = (sign === '-' ? -1 : 1) * (deD + deM / 60 + deS / 3600);
+  return { hr, raDeg, decDeg, vmag, bv };
+}
+
+const round = (value, decimals) => {
+  const scale = 10 ** decimals;
+  return Math.round(value * scale) / scale;
+};
+
+/** Build the committed pack from the raw catalogue text. */
+export function buildStarPack(text, { fetched = null } = {}) {
+  const stars = [];
+  let dropped = 0;
+  for (const line of text.split('\n')) {
+    if (line.trim() === '') continue;
+    const record = parseBscRecord(line);
+    if (!record) {
+      dropped += 1;
+      continue;
+    }
+    stars.push(
+      round(record.raDeg, 3),
+      round(record.decDeg, 3),
+      round(record.vmag, 2),
+      record.bv === null ? null : round(record.bv, 2),
+    );
+  }
+  return {
+    meta: {
+      source:
+        'Yale Bright Star Catalogue, 5th Revised Ed. (Hoffleit & Warren 1991), CDS V/50',
+      url: SOURCE_URL,
+      license: 'public domain',
+      fetched,
+      frame: 'ICRF/J2000, epoch 2000.0',
+      columns: ['raDeg', 'decDeg', 'vmag', 'bv'],
+      count: stars.length / 4,
+      dropped,
+    },
+    stars,
+  };
+}
+
+async function readCatalogue(file) {
+  if (!file) {
+    const response = await fetch(SOURCE_URL);
+    if (!response.ok) throw new Error(`${SOURCE_URL}: HTTP ${response.status}`);
+    return zlib
+      .gunzipSync(Buffer.from(await response.arrayBuffer()))
+      .toString('latin1');
+  }
+  const bytes = fs.readFileSync(file);
+  return (file.endsWith('.gz') ? zlib.gunzipSync(bytes) : bytes).toString(
+    'latin1',
+  );
+}
+
+async function main() {
+  const [file] = process.argv.slice(2);
+  const text = await readCatalogue(file);
+  const pack = buildStarPack(text, {
+    fetched: new Date().toISOString().slice(0, 10),
+  });
+  // One star per line keeps the JSON compact and the diff readable.
+  const rows = [];
+  for (let i = 0; i < pack.stars.length; i += 4)
+    rows.push(JSON.stringify(pack.stars.slice(i, i + 4)).slice(1, -1));
+  const body = `{"meta":${JSON.stringify(pack.meta)},\n"stars":[\n${rows.join(',\n')}\n]}\n`;
+  fs.mkdirSync(path.dirname(OUT), { recursive: true });
+  fs.writeFileSync(OUT, body);
+  console.log(
+    `wrote ${pack.meta.count} stars (${pack.meta.dropped} dropped) → ${path.relative(process.cwd(), OUT)} (${body.length} bytes)`,
+  );
+}
+
+if (process.argv[1] === fileURLToPath(import.meta.url)) await main();