IV.P126 — Bohr radius from iota_tau

The Bohr radius a_0 = R/(alpha_em m_n) (hbar/c) is fully determined by iota_tau and m_n, since both R = m_n/m_e (from the 10-link chain) and alpha_em ~ (8/15) iota_tau^4 are rational functions of iota_tau; the prediction matches the CODATA value a_0 ~ 5.292e-11 m.