Attractor-Driven Shape Universality in a Minimal Late-Time Interacting Dark Sector

This version presents the final fixed manuscript of the quadratic velocity-dependent interacting dark sector model. The paper establishes: • Existence of a narrow viable parameter band (76 configurations) • Late-time localized interaction behavior • Controlled linear growth suppression governed primarily by amplitude variation • Consistency with current RSD measurements without statistical preference over ΛCDM Observable-space dimensional reduction results are presented separately in a forthcoming companion work. This version focuses strictly on model viability and phenomenological consistency.