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

We present the Matter-Originated Spacetime Gradient Model (MOSGM), a covariant scalar-tensor modified gravity framework derived from a first-principles action. The model produces modified Einstein equations, a baryonic matter sourced scalar field equation, and a proof of total energy-momentum conservation. MOSGM adopts an exponential interpolation function that recovers both Newtonian and deep-MOND limits. A first-principles derivation of the External Field Effect (EFE) yields a positive-definite anisotropic diffusion tensor and a parameter-free environmental suppression prediction. A pilot observational consistency test using a stacked sample of 15 galaxy clusters shows weak statistical preference (Δ ln L = 1.63) relative to a standard phenomenological suppression law. The work is intended as a falsifiable framework with clear predictions for high-external-field environments rather than a complete alternative to ΛCDM. Version v4 includes statistical clarification, formatting corrections, and improved observational presentation. All analysis uses publicly available datasets (MCXC, NED, SIMBAD).