Keep the darker plume tuning and the tighter ice surface

Reverts the brighter fill: lit tops against near-black undersides, and the
higher-frequency surface undulation that separated the sheets more clearly.

7_ice_over.frag

@@ -31,7 +31,7 @@ vec2 atlasUV(vec2 world, float layer) {
 }
 // Smooth frozen surface: two octaves, no bubbles, no grain.
 float height(vec2 x) {
-  return 0.22 * fbm(x * 0.85 + 2.0) + 0.06 * fbm(x * 2.6 - 5.0);
+  return 0.15 * fbm(x * 1.3 + 2.0) + 0.05 * fbm(x * 3.7 - 5.0);
 }
 void main() {
   vec2 screen = gl_FragCoord.xy / u_res;