P0 / GHZ-Bridge: Protocol-Robust GHZ Advantage and the Hamming Decoherence Mechanism (RCD 2.0, Paper P0-a)

We provide a minimal analytic framework, entirely within standard open-quantum-system theory, that explains the protocol-robust GHZ advantage observed experimentally across heterogeneous quantum platforms. The framework rests on three results: (1) the Hamming Decay Theorem, showing that off-diagonal density matrix elements decay as lambdad_H under any local quantum channel; (2) a binomial derivation of the product-state fidelity F_prod = [(1+exp(-gamma*t))/2]^N under the IBM protocol; and (3) a unification of the two experimental regimes (IBM dephasing-dominated and IonQ amplitude-damping-dominated) via T2-blindness of the GHZ observable. A three-parameter noise model calibrated to IonQ 5-qubit data reproduces the selectivity pattern GHZ+, W-, Cluster- with correct signs. The structural invariant S(rho) captures the channel-independent topology underlying the advantage. Includes Appendix A: Revised Empirical Basis with cross-platform convergence data, selectivity tables, hardware threshold, and kill switches.