CDN-295: The Tangential Null

CDN-295 locks the geometry-control test paired with the CDN-294 flyby gate. If the GCFT dynamic channel is a first-order Doppler perturbation of Xi coherence modes, the leading response must follow signed radial velocity, not total speed. At pure tangential geometry, P_r = 0, the first-order radial shape vanishes and only a second-order tangential ceiling remains. For a 20 km/s tangential encounter with c_Xi = 900 km/s, the ceiling is 4.938e-4, only 2.271 percent of the 20 km/s radial reference. A speed-only first-order residual is therefore excluded from clean Channel-B counting.