Selective Probing of Boundary-Modified Vacuum Observables in Fixed-Gap Casimir Cavities

This paper proposes a fixed-gap comparative framework for selectively probing boundary-modified vacuum observables in Casimir cavities. Rather than treating the plate separation as a scanned external variable, the work treats the fixed gap as a structural parameter of the cavity and introduces (A_{A,B}(d)) as an operational differential asymmetry metric for comparing material-dependent Casimir responses within a common geometry. Engineered Casimir boundaries are treated not only as sources of correction to be modeled or subtracted, but as selective probes of vacuum observables. A structured set of candidate observables — including Casimir force, local electromagnetic density of states, two-point field correlators, quadrature noise, and quantum-emitter response — is organized by experimental accessibility into a three-tier hierarchy, with (A_{A,B}(d)) as the primary operational target. Three priority material comparisons are identified: Au/doped Si, Au/graphene, and normal-metal/superconductor. An explicit Lifshitz-theory evaluation for the Au/doped-Si pair at (T = 300 K) yields a spectral asymmetry growing from about 8% at 50 nm to nearly 19% at 200 nm, indicating that the limiting factor is expected to be the control of systematics rather than signal magnitude or detector sensitivity. The contribution is methodological. The paper does not claim the discovery of a new force or new physics beyond standard Casimir and cavity-QED phenomena. Instead, it provides a controlled comparative protocol, compatible with existing AFM-based, MEMS-based, and cavity-QED platforms, for using fixed-gap Casimir cavities as selective probes of boundary-modified electromagnetic vacuum response.