IV.T122 — Parity Bridge --- recalled from III.T07

Recall of Parity Bridge (III.T07): among the five sectors {D, A, B, C, omega}, only A has balanced polarity pol(A) = 0, nonzero coupling kappa(A;1) = iota_tau, and admits transitions interchanging chi_+ and chi_- components; no other sector satisfies all three conditions simultaneously.