IV.T178 — Z Boson Mass from Window Universality at +1039 ppm

M_Z = M_W/√(1−ι_τ+ι_τ²) ≈ 91,282 MeV at +1039 ppm from PDG 91,187.6±2.1 MeV. Uses on-shell relation M_Z = M_W/cos(θ_W) with tree-level Weinberg angle sin²θ_W = ι_τ(1−ι_τ). Precision limited by tree-level θ_W; NLO formula matches MS-bar scheme (0.23122 at −0.7 ppm) but requires radiative correction Δr for on-shell mass relation.