X72-12: Critical-Dual Cancellation Trade-Off

Continuing from Round 11 (the exact contraction, in L², of the dual generating functional and the backward dual equation), this round tests whether the critical dual spaces L^{3/2} and H^{-1/2} likewise admit an exact contraction. Proves that the L² contraction depends simultaneously on two structures — projection compatibility and transport antisymmetry: L^{3/2} retains the transport chain-rule cancellation but loses Leray-projection compatibility, while H^{-1/2} retains projection compatibility but loses transport commutativity; and, within two restricted classes — locally isotropic integral metrics and radial Hilbert-multiplier metrics — proves that L² is the unique metric retaining both, yet L² is not scale-critical. This round accordingly establishes a 'criticality/cancellation trade-off' and stops at STOP-C16 (the criticality/double-cancellation gap), explicitly stating this is a restricted no-go (nonlocal or relational functionals are not excluded), and hands off to the next round's question of whether the projected entropy gradient can be re-integrated back into a scalar functional.