feat: terrain-following cloud base

config.js

@@ -335,6 +335,14 @@ window.CONFIG = {
             start: 0.35,          // world units: clouds fade out very close to camera
             end: 2.5              // full density from this distance onward
         },
+        // Terrain-following cloud base. The DEM height grid is sampled in
+        // the cloud shader, so the cloud bottom rises locally to keep a
+        // clearance above the visible ground. 'bottom' remains the global
+        // floor for areas without terrain data.
+        groundFollow: {
+            enabled: true,        // false = flat cloud deck
+            clearanceM: 50        // metres above terrain, exaggerated with terrain
+        },
         // Coverage per weather level, 0 = clear, 1 = overcast. Only the
         // levels listed here override the built-in defaults
         // (sunny 0.20, cloudy 0.70, stormy 0.80, night 0.45). Live weather