fix(sky): keep the Sun and Moon, settle outside the ICRF tables

- Pass the custom sky box to the Viewer instead of `skyBox: false`, which
  also stopped CesiumWidget from creating the Sun and the Moon. Guarded by
  scripts/qa-sky-sun-moon.mjs.
- Outside the IAU 2006 tables (before 1975, after ~2050) the preload
  resolves without data; remember that span and stay on TEME instead of
  preloading and rendering every frame.
- The anime style no longer cel-shades the far plane, where four bands
  turned the faint Milky Way into blotches.
- Paint the planets out after the star-removing opening and with the
  ring median, so no brighter plateau is left; faces rebuilt.

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

src/tooling/skyAssets.paintOut.test.mjs

@@ -0,0 +1,64 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import sharp from 'sharp';
+import {
+  CUBE_FACES,
+  FACE_FILE_SUFFIX,
+  PLANETS,
+  cubeFaceDirection,
+  icrfToGalactic,
+} from '../../scripts/build-milky-way.mjs';
+import { raDecToUnitVector } from '../sky/stars.js';
+
+const SKY = new URL('../data/local_data/sky/', import.meta.url);
+// Allowed excess of the painted disc over the surrounding sky, in 8-bit
+// levels of the encoded face: the blur and dither move values by a few.
+const TOLERANCE = 4;
+
+test('painted-out planets leave no disc brighter than the sky around them', async () => {
+  const centres = PLANETS.map((planet) => ({
+    planet,
+    vector: raDecToUnitVector(planet.l, planet.b),
+    disc: [],
+    ring: [],
+  }));
+  for (const face of CUBE_FACES) {
+    const { data, info } = await sharp(
+      new URL(`milkyway_${FACE_FILE_SUFFIX[face]}.webp`, SKY).pathname,
+    )
+      .raw()
+      .toBuffer({ resolveWithObject: true });
+    const size = info.width;
+    for (let j = 0; j < size; j++) {
+      for (let i = 0; i < size; i++) {
+        const d = cubeFaceDirection(face, (i + 0.5) / size, (j + 0.5) / size);
+        const n = Math.hypot(...d);
+        const g = icrfToGalactic(d.map((x) => x / n));
+        for (const centre of centres) {
+          const cos =
+            g[0] * centre.vector[0] +
+            g[1] * centre.vector[1] +
+            g[2] * centre.vector[2];
+          const degrees = (Math.acos(Math.min(1, cos)) * 180) / Math.PI;
+          const k = (j * size + i) * info.channels;
+          const luma = (data[k] + data[k + 1] + data[k + 2]) / 3;
+          if (degrees <= centre.planet.radiusDeg - 0.5) centre.disc.push(luma);
+          else if (
+            degrees >= centre.planet.radiusDeg + 1 &&
+            degrees <= centre.planet.radiusDeg + 3.5
+          )
+            centre.ring.push(luma);
+        }
+      }
+    }
+  }
+  const median = (values) => values.sort((a, b) => a - b)[values.length >> 1];
+  for (const { planet, disc, ring } of centres) {
+    const discMean = disc.reduce((s, v) => s + v, 0) / disc.length;
+    const ringMedian = median(ring);
+    assert.ok(
+      discMean <= ringMedian + TOLERANCE,
+      `l=${planet.l} b=${planet.b}: disc mean ${discMean.toFixed(1)} vs surrounding sky ${ringMedian.toFixed(1)}`,
+    );
+  }
+});