Spacetime Geometric Memory as an Effective Dark Halo

General Relativity, in its weak-field limit, assumes the quasi-static gravitational response to matter is instantaneous. This work develops a constitutive framework in which spacetime has a finite geometric relaxation time, τ. The value of τ is inferred from SPARC rotation curves and then held fixed across independent tests involving weak-lensing shear profiles, gas-lensing offsets in cluster mergers, environmental peculiar velocities, and CAMB-lite cosmological consistency checks. The results suggest that part of the weak-field halo phenomenology usually mapped into unseen mass may instead correspond to residual spacetime geometry: curvature that has not yet relaxed to the instantaneous baryonic configuration.