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/tooling/skyAssets.test.mjs

@@ -0,0 +1,107 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import {
+  buildStarPack,
+  parseBscRecord,
+} from '../../scripts/build-bright-stars.mjs';
+import {
+  CUBE_FACES,
+  cubeFaceDirection,
+  directionToPlateCarree,
+  icrfToGalactic,
+} from '../../scripts/build-milky-way.mjs';
+import { raDecToUnitVector } from '../sky/stars.js';
+
+// Verbatim CDS V/50 records: HR 1, Sirius (HR 2491) and a nova with no J2000
+// position. Columns matter, so the padding is part of the fixture.
+const HR_1 =
+  '   1          BD+44 4550      3 36042          46           000001.1+444022000509.9+451345114.44-16.88 6.70  +0.07 +0.08         A1Vn               -0.012-0.018      -018      195  4.2  21.6AC   3';
+const SIRIUS =
+  '2491  9Alp CMaBD-16 1591  48915151881 257I   5423           064044.6-163444064508.9-164258227.22-08.88-1.46   0.00 -0.05 -0.03   A1Vm               -0.553-1.205 +.375-008SBO    13 10.3  11.2AB   4*';
+const NOVA =
+  '  92 NOVA 1572                                     B Cas                                                                                                                                            *';
+
+const close = (actual, expected, epsilon = 1e-9) =>
+  assert.ok(
+    Math.abs(actual - expected) <= epsilon,
+    `${actual} is not within ${epsilon} of ${expected}`,
+  );
+
+test('BSC5 records parse J2000 position, V magnitude and B−V', () => {
+  const sirius = parseBscRecord(SIRIUS);
+  assert.equal(sirius.hr, 2491);
+  close(sirius.raDeg, (6 + 45 / 60 + 8.9 / 3600) * 15);
+  close(sirius.decDeg, -(16 + 42 / 60 + 58 / 3600));
+  assert.equal(sirius.vmag, -1.46);
+  assert.equal(sirius.bv, 0);
+  const first = parseBscRecord(HR_1);
+  close(first.raDeg, (5 / 60 + 9.9 / 3600) * 15);
+  close(first.decDeg, 45 + 13 / 60 + 45 / 3600);
+  assert.equal(first.vmag, 6.7);
+  assert.equal(first.bv, 0.07);
+  assert.equal(parseBscRecord(NOVA), null);
+});
+
+test('the star pack is a flat, rounded column array that counts its drops', () => {
+  const pack = buildStarPack([HR_1, NOVA, SIRIUS, ''].join('\n'), {
+    fetched: '2026-10-19',
+  });
+  assert.deepEqual(
+    pack.stars,
+    [1.291, 45.229, 6.7, 0.07, 101.287, -16.716, -1.46, 0],
+  );
+  assert.equal(pack.meta.count, 2);
+  assert.equal(pack.meta.dropped, 1);
+  assert.equal(pack.meta.fetched, '2026-10-19');
+});
+
+test('cube faces use the WebGL major-axis layout', () => {
+  // Each face centre looks down its own axis …
+  const axes = {
+    positiveX: [1, 0, 0],
+    negativeX: [-1, 0, 0],
+    positiveY: [0, 1, 0],
+    negativeY: [0, -1, 0],
+    positiveZ: [0, 0, 1],
+    negativeZ: [0, 0, -1],
+  };
+  for (const face of CUBE_FACES)
+    assert.deepEqual(
+      cubeFaceDirection(face, 0.5, 0.5).map((v) => v + 0),
+      axes[face],
+    );
+  // … and the first texel row and column follow OpenGL ES 3.0 §3.8.10:
+  // on +X, s grows towards −Z and t towards −Y.
+  assert.deepEqual(cubeFaceDirection('positiveX', 0, 0), [1, 1, 1]);
+  assert.deepEqual(cubeFaceDirection('positiveZ', 1, 1), [1, -1, 1]);
+  assert.deepEqual(cubeFaceDirection('negativeY', 0, 1), [-1, -1, -1]);
+  assert.throws(() => cubeFaceDirection('up', 0, 0), RangeError);
+});
+
+test('plate carrée is centred on longitude 0, growing to the left, north up', () => {
+  assert.deepEqual(directionToPlateCarree(1, 0, 0), [0.5, 0.5]);
+  const sixHours = directionToPlateCarree(0, 1, 0);
+  close(sixHours[0], 0.25);
+  const eighteenHours = directionToPlateCarree(0, -1, 0);
+  close(eighteenHours[0], 0.75);
+  close(directionToPlateCarree(0, 0, 1)[1], 0);
+  close(directionToPlateCarree(0, 0, -1)[1], 1);
+  const twelveHours = directionToPlateCarree(-1, 0, 0);
+  assert.ok(twelveHours[0] === 0 || twelveHours[0] === 1);
+});
+
+test('the galactic rotation puts the Galactic Centre and pole where the IAU does', () => {
+  // Sgr A*-adjacent origin l = b = 0 at RA 266.405°, Dec −28.936° (J2000).
+  const centre = icrfToGalactic(raDecToUnitVector(266.40499, -28.93617));
+  close(centre[0], 1, 1e-6);
+  // North galactic pole at RA 192.85948°, Dec +27.12825°.
+  const pole = icrfToGalactic(raDecToUnitVector(192.85948, 27.12825));
+  close(pole[2], 1, 1e-6);
+  // ESO's panorama: the Large Magellanic Cloud (l ≈ 280.5°, b ≈ −32.9°) sits
+  // right of and below the centre.
+  const [u, v] = directionToPlateCarree(
+    ...icrfToGalactic(raDecToUnitVector(80.894, -69.756)),
+  );
+  close(u, 0.5 + (360 - 280.47) / 360, 2e-3);
+  close(v, 0.5 + 32.89 / 180, 2e-3);
+});