Add volumetric clouds3d renderer

Raymarches a horizontal cloud layer (28 steps, 4-octave 3D fBm).
Camera path is sinusoidal in altitude so the view dips into the
cloud deck at intervals for a 'fly through the edges' feel.
Density threshold from weather: sunny -> sparse, stormy -> overcast.
Renderer is selected via CONFIG.renderer ('clouds2d' or 'clouds3d').

config.js

@@ -76,7 +76,7 @@ window.CONFIG = {
     debug: 0,
     theme: 'ash',
     units: 'metric',
-    renderer: 'clouds2d',  // 'vanta' or 'clouds2d'
+    renderer: 'clouds3d',  // 'vanta', 'clouds2d', or 'clouds3d'
     language: 'de',
     timeFormat: 'DD. MMMM, YYYY ‒ HH:mm:SS',
     location: {