The Dark Energy Spectroscopic Instrument (DESI) 2024 Year 1 data release presents compelling, 3-sigma evidence that Dark Energy may not be a static cosmological constant (Lambda), but rather a dynamical fluid that evolves over cosmic time. Best-fit phenomenological models using the standard w0 wa parameterization suggest Dark Energy transitions from a phantom regime (w -1) today. While standard scalar-field models face unique mathematical challenges in establishing a first-principles mechanism for this precise "phantom crossing," we explore a novel perspective by modeling the spatial vacuum as a closed thermodynamic substrate governed by Symbiotic Infodynamic Equilibrium (SIE). We demonstrate that Dark Energy can be naturally understood not as an intrinsic vacuum energy, but as the macroscopic osmotic pressure required to continuously balance the universal thermodynamic ledger. By mapping this osmotic pressure directly to the cooling thermal history of the Cosmic Microwave Background (T proportional to a^-1) and the accelerating structural variance of late-stage galaxy clustering (variance proportional to a^2), we mathematically derive a rigorous, physical equation of state w_SIE(a). By explicitly calibrating the dimensionless thermodynamic coefficients (kappa_osm ~ 0.311, tau_land ~ 0.131) against empirical constraints, this equation organically reproduces the specific phantom-crossing evolution observed by DESI without violating early-universe Big Bang Nucleosynthesis constraints. Furthermore, it predicts a steep, non-linear divergence from the CPL parameterization at high redshifts, offering a pathway to transition the analysis of Dark Energy from statistical curve-fitting to strict thermodynamic hypothesis testing.