Reduce cloud structure: 2 layers, 3 octaves

- fbm3: 4 -> 3 octaves
- cloudDensity: 4 layers (base + mid + fine1 + fine2) -> 2 layers
  (base + mid at 20%)
Cost per pixel: 16 noise3 -> 6 noise3 per step, ~62% cheaper.
Visual: clouds still have secondary detail but the fine wispy
edges are gone — back to the cleaner silhouette.

vendor/clouds3d.js

@@ -90,23 +90,21 @@ var FRAG_SRC = [
     'float fbm3(vec3 p) {',
         'float v = 0.0;',
         'float a = 0.5;',
-        'for (int i = 0; i < 4; i++) {',
+        'for (int i = 0; i < 3; i++) {',
             'v += a * noise3(p);',
             'p = p * 2.03 + vec3(13.7, 7.1, 19.3);',
             'a *= 0.5;',
         '}',
         'return v;',
     '}',
-    // Density at a point in the cloud layer. Multi-layer fBm: base for
-    // the large puffs, mid layer for secondary structure, two fine
-    // layers for wispy edges. Combined gives richly textured clouds
-    // even at low weather density.
+    // Density at a point in the cloud layer. Two fBm layers: the
+    // base one at low frequency gives the large puffs, the detail
+    // layer at higher frequency breaks up the silhouette so clouds
+    // don't look like soft blobs.
     'float cloudDensity(vec3 p, float baseThresh) {',
         'float n = fbm3(p * 0.35 + vec3(0.0, 0.0, uTime * 0.06));',
         'float d = fbm3(p * 1.4 + vec3(uTime * 0.04, 7.3, uTime * 0.03));',
-        'float f1 = fbm3(p * 3.2 + vec3(2.1, uTime * 0.02, 5.7));',
-        'float f2 = fbm3(p * 6.5 + vec3(11.3, 4.7, uTime * 0.05));',
-        'n = n + d * 0.35 + f1 * 0.18 + f2 * 0.10;',
+        'n = n + d * 0.20;',
         // Cloud layer thickness: between y=-0.3 (deck bottom)
         // and y=1.5 (top), densest in the middle.
         'float h = clamp((p.y + 0.3) / 1.8, 0.0, 1.0);',