Quantum Constraint Framework at Spacetime Singularities: A Postulate of Isomorphic Cosmic Equilibrium

In brief: We identify the horizon Einstein tensor with an effective quantum surface stress $\Sigma_{\mu\nu}^{\text{QM}}$ sourced by horizon entropy. Enforcing the Bekenstein bound then yields a universal curvature ceiling $K_{\text{max}} = 1/16$, a finite interior cutoff, and a testable primordial-black-hole signal. (V1 published June 2026; V2.00 updated June 2026 with full numerical simulation suite). General Relativity predicts curvature divergence as $r \rightarrow 0$ in spherically symmetric spacetimes, while quantum unitarity forbids the non-unitary loss of information. We propose the Quantum Constraint Framework (QCF), wherein the classical Schwarzschild radius $r_s$ acts as a thermodynamic interface. Spacetime curvature at the horizon is sourced by the coarse-grained von Neumann entropy of horizon-localized modes. The Einstein tensor at the boundary is identified with an effective quantum surface stress tensor $\Sigma_{\mu\nu}^{\text{QM}}$ enforcing the Bekenstein entropy bound. Saturation of this informational bound yields a universal, finite curvature ceiling $K_{\text{max}} = 1/16$ (in Planck units), dynamically inducing an interior spatial cutoff $r_{\text{cut}} \propto r_s^{1/3}$. We present the complete mathematical derivation, define the underlying physical mechanism of Isomorphic Cosmic Equilibrium (ICE), and calculate micro-black-hole phenomenological signatures to establish empirical falsifiability.