A classical input not on XII-D's list

The held-fixed row of the ledger needs one theorem that XII-D did not hand over because it is not XII-D's: X-A's classical on-shell covariance of the regulated, switched scalar completion XA. It is proved on exactly the present sheet (same ramp, switch, plateau, Cauchy prescription, signature, and inverse normalization). We list it as a premise rather than cite it as comparison.