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 polarization — not as a derived artifact of matter loops, but as an intrinsic response sector encoded by an antisymmetric tensor field M_uv. This framework realizes a vanishing-background / finite-response principle: the vacuum carries no monopole or dipole content ( = 0), yet mediates a curvature-dependent electromagnetic susceptibility at tree level. Integrating out this microstructure generates curvature–photon operators reproducing the known GR+QED effective action structure, but with parametrically distinct coefficient scaling set by vacuum stiffness rather than electron-loop suppression. The theory predicts measurable gravitational birefringence in resonant-cavity experiments, opening a new terrestrial test window for vacuum stiffness dynamics. We further introduce a Baseline Neutrality Theorem implementing trace-modified gravitational coupling, which renders constant vacuum energy gravitationally inert through exact trace cancellation while preserving dynamical vacuum response. This removes ultraviolet sensitivity underlying the traditional zero-point energy gravitation paradox, leaving a residual integration constant not fixed by the present framework. The resulting theory separates baseline vacuum energy from dynamical vacuum polarization, preserves covariance and radiative stability within the effective field theory regime, and yields falsifiable predictions accessible to precision resonant-cavity experiments.