Vacuum Microstructure: Tree-Level Curvature-Photon Coupling and Exact Decoupling of Zero-Point Energy

We construct a covariant effective field theory in which vacuum exhibits dynamical polarizationnot as an emergent artifact of charged-matter loops, but as an intrinsic response sector encoded by an antisymmetric tensor field 𝑀 𝜇𝜈. The framework realizes a vanishing-background / finiteresponse principle: the vacuum carries no monopole or dipole content ⟨𝑀 𝜇𝜈 ⟩ = 0, yet mediates a curvature-dependent electromagnetic susceptibility at tree level. Integrating out the vacuum microstructure generates curvature-photon operators reproducing the known effective action structure of general relativity coupled to quantum electrodynamics, but with parametrically distinct coefficient scaling set by vacuum stiffness rather than electron-loop suppression. This leads to a polarization-dependent modification of photon propagation in weak gravitational fields and predicts a measurable birefringence signal in finite, quasi-local resonantcavity experiments under terrestrial gravity. We further establish a Baseline Neutrality Theorem implementing trace-modified gravitational coupling, which renders constant vacuum energy gravitationally inert through exact trace cancellation while preserving all dynamical vacuum response phenomena. This eliminates ultraviolet sensitivity associated with zero-point energy gravitation and is radiatively stable within the effective field theory regime. A residual integration constant remains as an observational input and is not fixed by the present framework. The resulting theory consistently separates baseline vacuum energy from dynamical vacuum polarization, maintains general covariance, and yields falsifiable quasi-local laboratory predictions in bounded regions, providing a concrete experimental pathway for probing vacuum stiffness dynamics and offering a controlled resolution of the vacuum-energy gravitation catastrophe without fine-tuning.