A Covariant Scalar-Field Foundation for the Cosmological Scaling of a_0

We derive the MOND acceleration scale a_0 as a dynamic consequence of cosmic horizon boundary conditions, a_0(z) = ξ c H(z). We demonstrate that the universal coupling ξ ≈ 0.049 is anchored to first principles via a tracking scalar field with slope α ≈ 0.0555, determined by the comoving Hubble-radius-to-sound-horizon ratio at recombination. We resolve observed scatter in ξ by unifying internal and external field effects (EFE) under a single informational drag parameter and provide kinematic evolution predictions consistent with the high-redshift v/σ ≈ 6.0 stability ratio. A key falsifiable prediction is the Vflat ∝ H(z)1/4 scaling, which preserves a redshift-invariant mass-velocity-size manifold.