CDN-292: The Rheology Wall

CDN-292 formalizes the GCFT high-drive Xi-field rheology wall. The dynamic channel remains first-order Doppler-linear at low radial drive, enters a nonlinear dissipative transition near D_Xi = |v_r|/v_star with v_star = 0.15 c_Xi, and freezes out at high drive. For the CDN-274 reference scale c_Xi = 900 km/s, the wall sits at v_star = 135 km/s, with the inferred window range 97.5-174 km/s for c_Xi = 650-1160 km/s. The claim is explicitly bounded: this is a subluminal coherence-response boundary, not a derivation of the special-relativistic speed limit, not faster-than-light transport, and not a replacement for c.