Constant X_BALANCED

pub const X_BALANCED: f64 = _; // 3.6015707105587516f64

x₀ = 4/(3α_ρ) ≈ 3.60 — balanced injection frequency.

Photon injection at x = x₀ produces zero net μ-distortion because the energy-per-photon exactly matches the mean energy of the background.

Reference: Chluba (2015), arXiv:1506.06582, Eq. 31