IV.T143 — Koide Q=2/3 from sigma-Symmetry

Any sigma-equivariant mass matrix M_l = [[a,b,0],[b,c,b],[0,b,a]] satisfies Koide Q=2/3 for its eigenvalues. Proof: sigma-odd eigenvector [1,0,-1] gives lambda_1=a; remaining eigenpairs lie on the democratic equilateral surface. Q=2/3 is an exact algebraic identity for any such matrix, independent of (a,b,c) values. Observed Q = 0.6666605 = 2/3 - 9.24 ppm is consistent with O(alpha^2) radiative deviation.