A Luminal, Symmetry-Grounded Dark Energy Mechanism from Kinetic Gravity Braiding (Paper 7 of the Physics Compiler Series)

Modern cosmology exhibits tensions in the inferred Hubble constant H0H_0H0 and late-time clustering amplitude S8S_8S8, while GW170817 imposes a strict luminality constraint cT=cc_T=ccT=c on modified gravity. This work applies the Physics Compiler audit framework to the post-GW170817 scalar–tensor landscape, enforcing a structural LUMINAL gate and symmetry-grounded admission. The resulting minimal luminal-safe candidate is shift-symmetric Kinetic Gravity Braiding (SS-KGB; cubic Horndeski with constant G4G_4G4 and G5=0G_5=0G5=0). We present the model ansatz, background closure via braiding friction, and a gate-based runtime audit (stability, asymptotic fate), and define a falsifiable discriminator for surveys: standard slip η≈1\eta \approx 1η≈1 with modified growth μ≠1\mu \neq 1μ=1 in the quasi-static regime. Supplementary materials include a drift-free J=0J=0J=0 branch algebraic solver script for reproducing benchmark outputs. “v1.1 adds ‘Predictions and Verification Roadmap’ section; no change to core claims.”