CDN-296: The Signed-Residual Protocol

CDN-296 closes the CDN-292 through CDN-296 rheology-control run by converting the flyby gate and tangential null into a single regression protocol. The locked predictor is the signed radial channel B_i = (v_r,i/c_Xi)/(1+(|v_r,i|/v_star)^2), with a second-order tangential ceiling T2_i <= (v_t,i/c_Xi)^2 and regression Delta_i = A B_i + C T2_i + b0 + epsilon_i. A frozen 21-case design containing radial, oblique, and pure tangential controls gives normalized shape error 1.000 for intercept-only, speed-even, and tangential-only models, and 0.000 for signed-radial models. This is a pre-registered shape test, not a measured flyby detection claim.