Unified Spacetime Fluid Cosmology: A Falsifiable Phenomenological Framework for Late-Time Cosmic Expansion and Structure Growth

Unified Spacetime Fluid (USF) cosmology is formulated as a falsifiable phenomenological framework for late-time cosmic expansion and structure growth. The central idea is to generalize the cosmological-constant sector into a spacetime-sector response while preserving the linear-GR limit. At the background level, the ΛCDM constant term is replaced by an effective geometric component Ω_S(a), with ΛCDM recovered as the constant limit. At the perturbative level, the framework introduces an effective spacetime-friction term X(a,k) in the matter growth equation while keeping μ = Σ = 1 and c_T = 1. This overview is intended to serve as a stable conceptual and notational reference. It summarizes the minimal USF construction, its relation to ΛCDM, its observational identity, and its falsifiability criteria. The report does not claim a completed fundamental theory; it defines a structured research program in which spacetime-sector dynamics can be tested against precision cosmological data. Detailed numerical scripts, processed data products, and likelihood definitions are not included in this overview archive and should be released separately when the corresponding technical analyses are finalized. Version v1.1 corrects public-file consistency and minor notation/layout issues. No scientific claims are changed relative to Version v1.0.