A Two-Step Quantum–Classical Threshold at 13.1–22.6 µg with Exact Ratio 3 from a Close-Packed Vacuum Lattice

We model the vacuum as a discrete face-centered-cubic (K=12) tensor network with Bell-pair bonds and use it to predict a two-step quantum-to-classical threshold for macroscopic center-of-mass superpositions. A reversible dispersive deformation of the center-of-mass mode sets in at msoft≈13.1μg, and coherence becomes geometrically unsustainable at mhard≈22.6μg. The two scales are separated by the exact, parameter-free ratio 3, fixed by the edge-to-circumradius ratio of the cuboctahedral triangular face. Gravitational-collapse models predict a single scale of the same order, so the distinctive, falsifiable content is the two-step structure and the exact 3 separation, testable by a mass scan across the window; the recent 16.2μg cat-state oscillator of Bild et al. falls between the thresholds, where coherence is not ruled out. The absolute window depends on the bond length L=4ln2ℓP≈1.665ℓP, fixed by one calibration against the Bekenstein–Hawking area law. This calibration and the Compton representability hypothesis—that a mass excitation remains coherent only while its reduced Compton wavelength is resolvable by the lattice—are stated model inputs rather than derivations, motivated by the Compton frequency internal clock of massive excitations, the mass cutoff generic to lattice-regularized field theories, and the total-mass dependence observed in composite-object interferometry. Open problems are stated explicitly.