A Unified Rank-2 Tensor Paradigm for Cosmic Inflaton Decay:\\ Resolving the $H_0$ and $S_8$ Anomalies via Causal Viscous Hydrodynamics

We present a novel, first-principles cosmological framework that unifies inflation, dark matter, and dark energy under a single primordial rank-2 tensor field $B_{\mu\nu}$. Following the cessation of inflation, a scale-separated decomposition splits the field into a frozen, long-wavelength background baseline ($B_{\mu\nu}^{(S)}$) driving dynamic, late-time cosmic acceleration, and a rapidly oscillating subhorizon mode ($B_{\mu\nu}^{(F)}$) that behaves macroscopically as cold dark matter. During the reheating epoch, explosive particle production via broad parametric resonance pumps energy density into the fast-oscillating sector; concurrently, this non-equilibrium injection introduces an inherent macroscopic fluid dissipation, establishing a microphysical origin for cosmic friction. By varying the ghost-free bimetric action, we derive the closed and modified Einstein Field Equation: $G_{\mu\nu}=8\pi G [T_{\mu\nu}^{(S)}+T_{\mu\nu}^{(F)}+\Pi(g_{\mu\nu}+u_{\mu}u_{\nu})]$, where the dynamic bulk viscous pressure $\Pi$ is governed by a causal, non-linear Israel-Stewart transport equation explicitly dependent on the changing global expansion rate: $\dot{\Pi} = -\frac{\Pi}{\tau_0} - \frac{3H\zeta}{\tau_0} - \frac{1}{2}\Pi ( 3H - \frac{\dot{\rho}_F}{\rho_F} + \frac{\dot{H}}{H} )$. Numerical integration of this autonomous system reveals a stable thermodynamic attractor tracking from $\Pi(z=10) = -258.17$ down to a dark-energy-like de Sitter floor of $\Pi(z=0) = -3.18$. This dynamic evolution smoothly increases the local Hubble expansion rate to resolve the $H_0$ tension, while the active early-universe viscous stress introduces a scale-dependent braking term ($\propto k^2$) that exponentially dampens the small-scale matter power spectrum $P(k)$, eliminating the $S_8$ over-clustering discrepancy. Finally, we establish precise observational benchmarks and falsifiability criteria for this model, detailing how its signature Gaussian-type power cutoff can be unambiguously detected through tomographic weak lensing by the \textit{Euclid Satellite} and the \textit{Vera C. Rubin Observatory} (LSST), mass-function limits on satellite dwarf galaxies by the \textit{Nancy Grace Roman Space Telescope}, and high-redshift 21cm intensity mapping by the \textit{Square Kilometre Array} (SKA).