More clouds + finer structure, switch default back to clouds3d

- WEATHER_DENSITY bumped: sunny 0.20->0.55, cloudy 0.70->0.85,
  night 0.45->0.65. Stormy stays at 1.0 (already maxed).
  Even 'sunny' now has visible cloud structure, not just clear sky.
- clouds3d: add a second fBm layer at 4x frequency for fine detail,
  blended at 18% into the base. Combined with the tighter smoothstep
  (0.24 -> 0.18) the cloud silhouettes have more interesting edges
  instead of soft blob shapes.
- Same density bump applied to clouds2d so swapping renderers
  doesn't change the visual weather.
- config.renderer: clouds2d -> clouds3d (user request)

vendor/clouds2d.js

@@ -122,10 +122,10 @@ function hex(rgb) {
 }
 
 var WEATHER_DENSITY = {
-    sunny: 0.20,
-    cloudy: 0.70,
+    sunny: 0.55,
+    cloudy: 0.85,
     stormy: 1.00,
-    night: 0.45
+    night: 0.65
 };
 
 // Load the noise image and upload it as a REPEAT-wrapped texture.