pub fn mu_shape(x: f64) -> f64Expand description
μ-distortion spectral shape: M(x) = (x/β_μ - 1) · e^x / (e^x - 1)²
The chemical potential distortion; crosses zero at x = β_μ ≈ 2.19. Normalized such that μ = 1.401 × (Δρ/ρ) for energy injection in μ-era. Reference: Chluba (2013), Eq. (5b).