CDN-294: The Flyby Gate

CDN-294 turns the CDN-293 sputter map into a clean low-drive flyby protocol for the GCFT dynamic Xi channel. The gate locks the dimensionless shape B(v_r) = (v_r/c_Xi) / (1 + (|v_r|/v_star)^2), using c_Xi = 900 km/s and v_star = 135 km/s. Inside the flyby band |v_r| <= 20 km/s, the response remains linear-elastic with maximum sputter index 0.02148 and maximum slope drift 0.01611 relative to the 10 km/s slope. The paper makes no detection claim; it locks the required sign, slope, and geometry rules for a valid flyby residual.