Variable: REF_GAS_DENSITY

const REF_GAS_DENSITY: 100 = 100;

Column-integrated effective density of the gas envelope (kg/m³).

Physically the envelope density varies with altitude (scale-height collapse near the surface, near-vacuum at the top). The lib treats the envelope as a single equivalent volume for the core/shell split, so this constant captures the "average density that would fill the same volume with the same mass". Tuned so Jupiter-like bodies (gasMassFraction ≈ 0.93) land near coreRadiusRatio ≈ 0.2.