V.T277 — Generator Orbit Suppression: iota_tau^15 from sigma-Involution

Each of 5 generators contributes iota_tau^dim(tau^3) = iota_tau^3 via sigma-involution traversing 3 independent fibration directions. Full Z/5Z orbit gives iota_tau^(3x5) = iota_tau^15 = (iota_tau^3)^W_3(4). Parallels SA-i mod-3 for strong CP.