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/clouds3d.js

@@ -117,16 +117,18 @@ var FRAG_SRC = [
     // Star field: project ray onto a far sphere, hash-grid for
     // positions. Visible above the horizon at night with a small
     // twinkle.
-    'float starField(vec3 r) {',
-        'vec3 sp = r * 220.0;',
-        'vec3 sf = floor(sp);',
-        'vec3 sfp = fract(sp) - 0.5;',
-        'float h = hash3(sf);',
-        'float starD = length(sfp);',
-        'float mask = step(0.985, h) * smoothstep(0.35, 0.0, starD);',
-        'float twinkle = 0.55 + 0.45 * sin(uTime * 3.0 + h * 47.0);',
-        'return mask * twinkle;',
-    '}',
+'float starField(vec3 r) {',
+            'vec3 sp = r * 220.0;',
+            'vec3 sf = floor(sp);',
+            'vec3 sfp = fract(sp) - 0.5;',
+            'float h = hash3(sf);',
+            'float starD = length(sfp);',
+            'float mask = step(0.985, h) * smoothstep(0.35, 0.0, starD);',
+            // Slow, subtle twinkle (0.5 Hz, ±15%). 3 Hz was too fast
+            // and looked like static instead of gentle scintillation.
+            'float twinkle = 0.85 + 0.15 * sin(uTime * 0.5 + h * 47.0);',
+            'return mask * twinkle;',
+        '}',
     'void main() {',
         'vec2 uv = (gl_FragCoord.xy / uResolution.xy) * 2.0 - 1.0;',
         'uv.x *= uResolution.x / uResolution.y;',
@@ -184,7 +186,12 @@ var FRAG_SRC = [
         // Night darkening — driven by sun altitude, not uDayProgress
         // (uDayProgress is the 0..1 clock progress through 24h, which
         // is not the same as 'is it night').
-        'col = mix(col, mix(uBg, col, 0.4), nightFactor * 0.45);',
+        // Only darkens the sky, NOT the clouds — we want them to stay
+        // moon-lit (their colour came from uLight) while the sky around
+        // them goes dark. The cloud mask 'cloud' tells us how much of
+        // this pixel is cloud, so fully cloudy pixels keep their colour.
+        'float skyDarkness = nightFactor * 0.85;',
+        'col = mix(col, col * (1.0 - skyDarkness), 1.0 - cloud);',
         'gl_FragColor = vec4(col, 1.0);',
     '}'
 ].join('\n');