Constant KAPPA_C

pub const KAPPA_C: f64 = _; // 2.1418515045023785f64

κ_c = 3 ∫x³ M(x) dx / G₃ = 12/β_μ − 9G₂/G₃ ≈ 2.1419

Derived analytically using ∫₀^∞ xⁿ eˣ/(eˣ−1)² dx = n·G_{n−1}: ∫x³ M(x) dx = ∫x³ (x/β_μ − 1) g_bb/x dx = (4G₃/β_μ) − 3G₂ κ_c = 3[(4G₃/β_μ) − 3G₂]/G₃ = 12/β_μ − 9G₂/G₃