An Effective Field-Theoretic Extension of General Relativity: Viscoelastic Spacetime Dynamics and High-Redshift Observational Signatures

General Relativity (GR) serves as the foundational framework for modern gravitation, demonstrating exceptional consistency across local and intermediate cosmological scales. In this work, we propose a systematic, field-theoretic refinement of GR by incorporating effective viscoelastic responses into the spacetime fabric as higher-order constitutive corrections. Beginning from a modified least-action principle, we step-by-step derive the exact gravitational field equations without introducing ad-hoc phenomenological parameters. We prove that in the static and non-memory limits, the field equations continuously reduce to Einstein's field equations, ensuring full compatibility with standard test bounds. To explore observational implications, we apply this framework to the expansion history H(z) and evaluate its signatures against high-redshift observational data, including observations from the James Webb Space Telescope (JWST) at z > 8. Finally, we outline how this foundational viscoelastic framework provides a robust theoretical basis for resolving late-time cosmological discrepancies, such as the H0 tension, in future work.