MOSGM: A Covariant Scalar-Tensor Theory with Exponential External-Field Suppression

We present the Matter-Originated Spacetime Gradient Model (MOSGM), a covariant scalar-tensor theory derived from a first-principles action principle. The framework produces modified Einstein equations, a baryonic-matter-sourced scalar field equation, and a proof of total energy-momentum conservation. An exponential interpolation function recovers deep-MOND scaling and Newtonian gravity in appropriate limits. The External Field Effect is derived perturbatively, yielding a positive-definite anisotropic diffusion tensor. Tested against a 15-cluster stacked sample, MOSGM achieves 0.34σ tension while the standard quadratic law shows 1.84σ tension.