Fly mostly above the clouds with brief dips

Camera altitude is now driven by pow(sin,5) so the view stays at
y=2.6 most of the time and only briefly dips to y=0.9 to skim the
top of the cloud deck. Marching also skips the empty gap between
camera and cloud ceiling when looking down from above.

vendor/clouds3d.js

@@ -101,29 +101,28 @@ var FRAG_SRC = [
     'void main() {',
         'vec2 uv = (gl_FragCoord.xy / uResolution.xy) * 2.0 - 1.0;',
         'uv.x *= uResolution.x / uResolution.y;',
-        // Camera path: slow drift in X, sinusoidal altitude.
-        // The altitude dip drives the "fly through cloud edges" effect.
+        // Camera path: slow drift in X, mostly above the cloud deck.
+        // pow(...,n) shapes the sin into short dips, so the view spends
+        // most of its time at cruising altitude and only briefly skims
+        // the top of the cloud layer.
         'float drift = uTime * uSpeed * 0.45;',
-        'vec3 ro = vec3(',
-            'drift,',
-            // Altitude oscillates: above (1.5), at cloud-top (0.6), inside (0.1)
-            '1.5 - 1.4 * (0.5 + 0.5 * sin(uTime * 0.07)),',
-            'drift * 0.7',
-        ');',
-        // Ray direction: gentle downward tilt so we see the cloud deck
-        // and the sky horizon at once.
-        'vec3 rd = normalize(vec3(uv.x * 0.55, -0.18 + uv.y * 0.25, 1.0));',
+        'float diveT = pow(0.5 + 0.5 * sin(uTime * 0.10), 5.0);',
+        'float camY = mix(2.6, 0.9, diveT);',
+        'vec3 ro = vec3(drift, camY, drift * 0.7);',
+        // Ray direction: tilt down a bit more so the cloud deck fills
+        // the lower frame when we're above it.
+        'vec3 rd = normalize(vec3(uv.x * 0.55, -0.35 + uv.y * 0.30, 1.0));',
         // Sky gradient based on ray Y (background).
         'float skyT = clamp(rd.y * 0.7 + 0.5, 0.0, 1.0);',
         'vec3 col = mix(uBg, uSky, skyT);',
         // Density threshold: weather 'sunny' -> high (sparse clouds),
         // 'stormy' -> low (overcast deck).
         'float baseThresh = mix(0.72, 0.40, uDensity);',
-        // March the cloud layer (bounded in Y).
-        'float t = 0.0;',
+        // Skip the gap between camera and cloud top so we don't waste
+        // steps marching through clear sky. ray-Y = 2.0 = cloud ceiling.
+        'float t = (ro.y > 2.0 && rd.y < -0.001) ? (ro.y - 2.0) / -rd.y : 0.0;',
         'float stepSize = 0.85;',
         'float trans = 1.0;',
-        // Pre-compute ray entry/exit on cloud layer for early out
         'for (int i = 0; i < 18; i++) {',
             'vec3 p = ro + rd * t;',
             'if (p.y > 2.0 || p.y < -0.4) break;',