Tune clouds3d for low-end GPUs: 18 steps, 3 octaves

Reduce raymarch iterations from 28 to 18 and fBm octaves from
4 to 3 (with step size 0.55->0.85 to preserve cloud coverage).
Per-pixel cost drops by ~55% while visual density stays similar.

vendor/clouds3d.js

@@ -44,7 +44,9 @@ var VERT_SRC = [
 ].join('\n');
 
 // Volumetric raymarch through a horizontal cloud layer.
-// Cheap 3D value noise + 4-octave fbm. 28 raymarch steps.
+// Cheap 3D value noise + 3-octave fbm. 18 raymarch steps.
+// Tuned for low-end hardware: fewer steps + fewer noise samples
+// per step keep the frame within budget on integrated GPUs.
 // Camera path is sinusoidal so the view occasionally dips
 // into the cloud deck for the "fly through the edges" feel.
 var FRAG_SRC = [
@@ -79,7 +81,7 @@ 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;',
@@ -119,10 +121,10 @@ var FRAG_SRC = [
         'float baseThresh = mix(0.72, 0.40, uDensity);',
         // March the cloud layer (bounded in Y).
         'float t = 0.0;',
-        'float stepSize = 0.55;',
+        'float stepSize = 0.85;',
         'float trans = 1.0;',
         // Pre-compute ray entry/exit on cloud layer for early out
-        'for (int i = 0; i < 28; i++) {',
+        'for (int i = 0; i < 18; i++) {',
             'vec3 p = ro + rd * t;',
             'if (p.y > 2.0 || p.y < -0.4) break;',
             'float d = cloudDensity(p, baseThresh);',