We propose a symmetry-driven cosmological framework in whichultraviolet (UV) and infrared (IR) corrections are organized by a single Scaling–Duality symmetry, H → ˜H ≡ H2∗/H, together with ahorizon-thermodynamic expansion of the correction sector. Writingthe modified constraint as H2 = (8πG/3)ρm + f(H) with ρm ∝ a−3,we derive a single autonomous “Master ODE” for the expansion history, H′ = −3(H2 −f(H))/(2H −f′(H)) with N ≡ lna. We adopta minimal publishable truncation—a cosmological-constant-like piece(n = 2), a dual pair (n = 1,3) with coefficient pairing c3 = κc1 (κ =±1), and a logarithmic running term γ—and present a referee-robustphase-space analysis. In particular, we prove a universal attractor result: every nondegenerate de Sitter fixed point in the E ≡ H/H∗ flow isautomatically attractive with eigenvalue λ = −3 in N time. We isolatethe only sharp background pathology (branch points 2E −F′(E) = 0)and provide a simple admissibility test for singularity-free evolution.A reproducible scan protocol (with explicit numerical routines) and aminimal Python implementation are provided.