An exact profile-robust diffusion-decoherence product bound in two-coupling postquantum classical gravity

This preprint derives a coupling-parameter-free diffusion–decoherence product floor from the static scalar-sector coefficients of a two-coupling linearised postquantum classical gravity model. On the positive-semidefinite domain, D_diff D_dec^(0) >= (25/9) 8 pi^2 G_N^2, with equality only at beta/alpha=1/6. The paper transfers the result through positive source profiles, states an explicitly conventioned low-frequency acceleration-noise times decoherence-rate exclusion law, and proves a conditional spectral-overlap theorem for arbitrary momentum running within the same two-operator coefficient map. The result is model-specific and is not a universal theorem for classical–quantum gravity. This version is a preprint and has not undergone independent external validation. Verification of the source coefficients and SI/PSD normalisation remains an explicit next step before journal submission.