IV.T177 — Precision W Boson Mass from Window Universality at −0.5 ppm

M_W = (17/5)·ι_τ⁻³·m_n·[1+(5/17)·α·ι_τ²] = 80,369.2 MeV at −0.5 ppm from PDG 80,369.2±1.3 MeV. Consolidates the ch27 Window Universality result (IV.T140) as a standalone precision W mass prediction. Zero free parameters: W₃(3)=17 (CF window a₃+a₄+a₅), W₃(4)=5 (a₄+a₅+a₆), α=(121/225)ι_τ⁴, m_n input anchor. Supersedes tree-level IV.T54 (~10% off).