Galactic Sector Completion: No-Go Theorems and Kinetic Screening

We prove three no-go theorems establishing that the minimal RCD action and its two simplest extensions cannot produce O (a₀) galactic effects. We then construct Completion B', the minimal kinetic extension K (X) = X + X²/X_* with conformal matter coupling A (C) = 1 + beta*f (C), that restores galactic phenomenology through k-mouflage screening. The theory is ghost-free, gradient-stable, and subluminal. The screening mechanism produces a universal transition acceleration aₜrans independent of system mass. The viability window for X_* spans ~15 orders of magnitude. Five independent observational kill switches are identified.