atmosphere: add physical Rayleigh sky scattering

config.js

@@ -313,6 +313,8 @@ 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)
+        physicalSky: true,        // Rayleigh sky scattering instead of pure theme gradient
+        rayleighGain: 8.0,        // exposure for the physical sky term
         distanceFog: false,       // legacy aerial perspective is off
         model: {
             enabled: false        // volumetric atmosphere/fog removed