Permanent above-clouds cruise + more structure

- Remove dive component from camY. Was mix(2.0, 1.2, diveT)
  (occasional dips into cloud top); now fixed at 2.2, always above
  the y=1.5 cloud ceiling. Cloud deck always visible below.
- WEATHER_DENSITY bumped again: sunny 0.55->0.65, cloudy 0.85->0.90,
  night 0.65->0.75. Even 'sunny' is now a partly-cloudy sky.
- clouds3d: third fBm detail layer at 3.2x frequency (was 2 layers).
  Contributions: base 1.0, mid 0.30, fine 0.12. Combined with the
  tighter smoothstep (0.18 -> 0.16) the silhouettes have wispier,
  more interesting edges instead of soft blobs.
- Same density bump in clouds2d for consistency.

vendor/clouds2d.js

@@ -122,10 +122,10 @@ function hex(rgb) {
 }
 
 var WEATHER_DENSITY = {
-    sunny: 0.55,
-    cloudy: 0.85,
+    sunny: 0.65,
+    cloudy: 0.90,
     stormy: 1.00,
-    night: 0.65
+    night: 0.75
 };
 
 // Load the noise image and upload it as a REPEAT-wrapped texture.