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 ΣμνQM sourced by horizon entropy. Enforcing the Bekenstein bound then yields a universal curvature ceiling Kₘₐₓ = 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 → 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 rₛ 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 ΣμνQM enforcing the Bekenstein entropy bound. Saturation of this informational bound yields a universal, finite curvature ceiling Kₘₐₓ = 1/16 (in Planck units), dynamically inducing an interior spatial cutoff rcut ∝ rₛ1/3. 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.