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).