Paper 4 - Phenomenological Interface and Observable Signatures

Paper 4 of the Abebe Carrier Framework under Master Map II. Derives observable consequences: stochastic gravitational-wave background (mHz–Hz peak from residual π₃(S³) defect network), high-scale gauge coupling deviations, parity-violating cosmological birefringence, and unified geometric gravity–SM interface. All predictions are parameter-free and falsifiable by LISA, Einstein Telescope, and CMB-S4. Part of the complete series (Papers 0–5) with Global Closure Appendices A–G. (Keywords: Carrier Framework, Master Map II, stochastic GW background, cosmological birefringence, defect network, falsifiable predictions)