This paper proposes a Unified Field Theory of Phygital Space, positing that contemporary reality is not a dichotomy of "online" and "offline," but a unified ontological manifold of irreducible but coupled dimensions. We formalize Phygital Space as a sheaf over a topological site composed of the Physical (U), Networked Digital (D), and Networked Social (S) dimensions, grounded in Informaticity -- the triune capacity to compute, communicate, and control -- and instantiated through Platforms. We develop a rigorous framework incorporating Finsler geometry to model the inherently asymmetric costs of cross-dimensional interaction. We define Ontological Mass (mu) as a tensor quantity encoding resistance to change across coupled dimensions, and introduce autopoietic dynamics to account for the endogenous agency of persons, algorithms, and social formations. We propose a non-equilibrium thermodynamic model where economic value is negentropy generated by platforms acting as dissipative structures. We introduce a theory of Temporal Shear formalized through Lie derivatives to explain the pathologies of modern time. The theory is empirically validated through a longitudinal analysis of the Chinese e-commerce ecosystem (1999-2025), modeling the dimensional trajectories of Taobao, JD . com, Pinduoduo, and Douyin across twenty-five years of evolution. We extend the framework to a post-human ecology of Synthetic Agents and articulate the normative implications for platform governance and human flourishing.