clouds: add new spherical cloud pass using terrain distance occlusion

config.js

@@ -143,6 +143,7 @@ window.CONFIG = {
     // World units (1 unit ~ 12.9 km). Keep 'top' below the lowest flight
     // altitude so aircraft fly above the deck.
     clouds: {
+        enabled: true,
         top: 1.0,                 // ~12.9 km
         bottom: -0.2,             // ~-2.6 km (deck base slightly below terrain)
         // Coverage per weather level, 0 = clear, 1 = overcast. Only the