fix(sky): preserve dark model lighting and address review

Merge origin/main and retain both format-scope additions.
Give StarrySky a separate black environment cube map and release it on destroy.
Guard anime updates when StarrySky is absent, require a visible Normal sky
before anime QA, and compare a lit airplane against black and white controls.
Document the sky background contract and its verification.

Co-Authored-By: GPT-6 (Codex) <[email protected]>

scripts/qa-sky-model-lighting.mjs

@@ -0,0 +1,147 @@
+#!/usr/bin/env node
+/**
+ * Lit airplane regression: StarrySky must match a black environment baseline.
+ * A white environment is a positive control: it must visibly brighten the
+ * same model. Each case uses a fresh scene and dynamic environment map.
+ * Usage: node scripts/qa-sky-model-lighting.mjs [--url http://localhost:4333/]
+ */
+import assert from 'node:assert/strict';
+import puppeteer from 'puppeteer';
+
+const i = process.argv.indexOf('--url');
+const base = i > 0 ? process.argv[i + 1] : 'http://localhost:4333/';
+const url = new URL('/__qa_sky_model__', base);
+const browser = await puppeteer.launch({
+  headless: true,
+  args: ['--no-sandbox', '--ignore-gpu-blocklist'],
+});
+try {
+  const page = await browser.newPage();
+  page.on('pageerror', (error) => console.log('PAGEERR', error.message));
+  await page.setRequestInterception(true);
+  page.on('request', (request) => {
+    if (request.url() === url.href)
+      request.respond({
+        contentType: 'text/html',
+        body: '<!doctype html><html><body></body></html>',
+      });
+    else if (new URL(request.url()).origin !== url.origin) request.abort();
+    else request.continue();
+  });
+  await page.goto(url.href);
+  const results = await page.evaluate(async () => {
+    window.CESIUM_BASE_URL = '/cesium/';
+    // Use StarrySky's exact Vite dependency URL, including its version query.
+    const source = await fetch('/src/sky/starrySky.js').then((r) => r.text());
+    const cesiumUrl = source.match(
+      /from ["']([^"']*\/cesium\.js[^"']*)["']/,
+    )?.[1];
+    if (!cesiumUrl) throw new Error('run against the Vite dev server');
+    const C = await import(cesiumUrl);
+    const { StarrySky } = await import('/src/sky/starrySky.js');
+    const time = C.JulianDate.fromIso8601('2026-10-19T00:00:00Z');
+    const results = {};
+    for (const kind of ['black', 'starry', 'white']) {
+      const canvas = document.createElement('canvas');
+      canvas.width = 400;
+      canvas.height = 300;
+      document.body.append(canvas);
+      const scene = new C.Scene({ canvas });
+      scene.rethrowRenderErrors = true;
+      let cubeMap;
+      try {
+        scene.backgroundColor = C.Color.BLACK;
+        scene.light = new C.DirectionalLight({
+          direction: new C.Cartesian3(1, 0, 0),
+          intensity: 0.2,
+        });
+        if (kind === 'starry') {
+          scene.skyBox = new StarrySky({ requestRender() {} });
+          scene.skyBox._loading = true; // visible sky is outside this probe
+        } else {
+          const value = kind === 'white' ? 255 : 0;
+          const face = {
+            width: 1,
+            height: 1,
+            arrayBufferView: new Uint8Array([value, value, value, 255]),
+          };
+          cubeMap = new C.CubeMap({
+            context: scene.context,
+            source: Object.fromEntries(
+              [...C.CubeMap.faceNames()].map((name) => [name, face]),
+            ),
+          });
+          scene.skyBox = { _cubeMap: cubeMap, update() {}, destroy() {} };
+        }
+        const position = C.Cartesian3.fromDegrees(0, 0, 10000);
+        const model = await C.Model.fromGltfAsync({
+          url: '/models/airplane.glb',
+          modelMatrix: C.Transforms.eastNorthUpToFixedFrame(position),
+        });
+        scene.primitives.add(model);
+        scene.camera.lookAt(position, new C.HeadingPitchRange(0.4, -0.5, 45));
+        let framed = false;
+        for (let frame = 0; frame < 240; frame++) {
+          scene.initializeFrame();
+          scene.render(time);
+          if (model.ready && !framed) {
+            scene.camera.viewBoundingSphere(
+              model.boundingSphere,
+              new C.HeadingPitchRange(
+                0.4,
+                -0.5,
+                model.boundingSphere.radius * 3,
+              ),
+            );
+            framed = true;
+          }
+          await new Promise(requestAnimationFrame);
+        }
+        if (!model.ready || !model.environmentMapManager.radianceCubeMap)
+          throw new Error(`${kind}: model or dynamic lighting did not load`);
+        scene.initializeFrame();
+        scene.render(time);
+        const gl = scene.context._gl;
+        const px = new Uint8Array(canvas.width * canvas.height * 4);
+        gl.readPixels(
+          0,
+          0,
+          canvas.width,
+          canvas.height,
+          gl.RGBA,
+          gl.UNSIGNED_BYTE,
+          px,
+        );
+        let total = 0;
+        let visible = 0;
+        for (let p = 0; p < px.length; p += 4) {
+          const luma = 0.299 * px[p] + 0.587 * px[p + 1] + 0.114 * px[p + 2];
+          total += luma;
+          if (luma > 1) visible++;
+        }
+        results[kind] = { total, visible };
+      } finally {
+        scene.destroy();
+        cubeMap?.destroy();
+        canvas.remove();
+      }
+    }
+    return results;
+  });
+  console.log(JSON.stringify(results));
+  assert.ok(results.black.visible > 100, 'the lit model must be visible');
+  assert.ok(
+    Math.abs(results.starry.total - results.black.total) <=
+      results.black.total * 0.01,
+    'StarrySky must match black environment lighting within 1%',
+  );
+  assert.ok(
+    results.white.total > results.black.total * 1.03,
+    'white environment control must brighten the model by at least 3%',
+  );
+  console.log(
+    'PASS: lit airplane matches black sky; white control is brighter',
+  );
+} finally {
+  await browser.close();
+}