Constraint-Layer Structure of Black Hole Interiors Under Minimal Physical Assumptions

We develop a constraint-based framework for black-hole interior dynamics under a minimal set of physical assumptions: (i) the validity of a local classical spacetime description, (ii) effective finite local relaxation within the interior matter, (iii) a separation between microscopic interaction scales and the curvature scale, and (iv) the absence of sustained external driving. Within this domain, we derive parametric constraints on admissible macroscopic behavior without specifying the underlying microscopic composition. Transport processes define a diffusive smoothing length below which spatial gradients are efficiently erased over a macroscopic time. Relaxation suppresses anisotropic stress and non-equilibrium deviations, damping inhibits sustained coherent macroscopic oscillation, and the absence of sustained driving prevents persistent far-from-equilibrium structure from being continually regenerated. The resulting constraint corridor is therefore not a claim that all macroscopic structure is excluded by diffusion alone. Rather, it is a necessary-condition framework: any persistent macroscopic structure must be supported by an explicit stabilizing channel, while unsupported anisotropy, non-equilibrium behavior, and coherent dynamics are dynamically suppressed under the stated assumptions. We further show that externally distinguishable interior structure is limited by the combined effects of causal propagation, damping, and model-dependent coupling to exterior observables. The results are conditional and do not apply in regimes where the underlying assumptions fail, such as collisionless systems, sustained driven systems, nonlocal regimes, or quantum-gravity dominated domains. Rather than proposing a specific interior model, this work establishes necessary conditions that admissible models must satisfy within a well-defined physical domain. Version 2.0 note: This version revises the interpretation of the transport/mixing scale. The diffusive scale is treated as a smoothing length rather than a universal upper bound on persistent structure. Persistent macroscopic structure is now treated as admissible only when an explicit stabilizing support channel is supplied and remains compatible with the remaining relaxation, damping, causal, and observational constraints.