atmosphere: add initial volumetric atmosphere shell

config.js

@@ -305,6 +305,15 @@ window.CONFIG = {
         albedoGain: 1.0,          // theme share on surface colours
         skyGain: 0.45,            // diffuse skylight share on surfaces
         airGain: 0.55,            // Rayleigh airlight strength (blue-biased)
+        distanceFog: false,       // legacy aerial perspective is off
+        model: {
+            enabled: true,
+            heightKm: 35.0,       // full atmosphere shell around the planet
+            scaleHeightKm: 8.5,   // density falls exponentially with altitude
+            density: 0.12,        // base optical density
+            grayTau: 1.5,         // high optical depth becomes grey fog
+            gain: 2.0             // scattering brightness
+        },
         hazeResidual: 0.30,       // surface colour surviving at the full fog distance
         cloudBrightness: 1.0,     // cloud albedo (theme hue, brightness from the model)
         cloudHaze: 0.22,          // share of the ground haze applied to the deck