A Falsifiable Geometric-Response Transition Benchmark for Late-Time Cosmic Acceleration

This working paper timestamps a conservative IVG/GMR-inspired geometric-response transition benchmark for the late-time cosmic acceleration epoch. It does not claim a first-principles calculation of the dark-energy amplitude or a complete relativistic theory. It distinguishes the standard LCDM acceleration redshift from the response-switch center z_R, defines a phenomenological branch-transition ansatz centered near z_R ~ 0.63, and gives falsifiable observational targets in H(z), Type-Ia supernova residuals, BAO distances, and growth/lensing consistency.