We present a reproducible tomographic parameter scan of a constrained 4×4 discrete grid model exhibiting a sharp critical phase inversion under monotonic variation of a single control parameter ϵ. The model is intentionally constructed as a toy system with centralized stress amplification, allowing clear identification of a localized overload threshold. A deterministic phase inversion is observed near ϵ ≈ 0.00724–0.00725, robust under fixed model assumptions. No physical constants are derived; instead, the study demonstrates how discrete geometric constraints alone can generate sharp stability boundaries.