Conditional Structural Decoupling on the Admissible Departed-Regime Window

We study the stochastic partial differential equation governing the SFIT capacity engine, in which the effective mass of the field is modulated by a microscopic coupling gg through a capacity-regulated pump. Under the departed-regime hypotheses—⟨C_loc⟩ ≫ C_max with bounded relative fluctuations σ_C/⟨C_loc⟩ ≈ 0.128 (empirically observed across a 40-fold variation in the damping ratio)—we prove that the pump modulation is exponentially suppressed and the microscopic coupling g is asymptotically decoupled from the stationary dynamics. The decoupling is established on a restricted window μ_C ≤ (1−δ)μ_C^⋆ within the departed regime, which we conjecture—but have not verified—encompasses the numerically accessible parameters. We identify the structural limitation of the two-sided sub-Gaussian tail bound and propose a one-sided large-deviation principle as the natural path to unconditional asymptotic decoupling. These results provide a rigorous bridge between empirical observations of the departed regime and the analytical understanding of the SFIT capacity engine's universal behavior.