Dark night sky + slow star twinkle + moonlit clouds

clouds3d:
- Star twinkle: 3 Hz ±45% -> 0.5 Hz ±15% (gentle scintillation
  instead of static-like flickering)
- Night darkening: applied only to sky pixels via the cloud mask,
  not to clouds. The darkening factor jumped 0.45 -> 0.85 so the
  night sky really does go black around moonlit clouds. Clouds
  keep their uLight-tinted colour so they remain visible.

clouds2d:
- Same darkening approach but via multiplicative factor (no cloud
  mask in the 2D shader): col *= 1 - nightFactor * 0.7

vendor/clouds2d.js

@@ -110,7 +110,9 @@ var FRAG_SRC = [
         // Night darkening, driven by sun altitude (not uDayProgress
         // which is a 24h clock progress, not "is it night")
         'float nightFactor = smoothstep(0.05, -0.15, uSunAlt);',
-        'col = mix(col, mix(uBg, col, 0.4), nightFactor * 0.45);',
+        // Stronger than clouds3d because there's no cloud-mask
+        // blending here — just multiply.
+        'col *= 1.0 - nightFactor * 0.7;',
         'gl_FragColor = vec4(col, 1.0);',
     '}'
 ].join('\n');