CDN-274: The Last Debt

Resolves D1, the final open theory debt: the v_r-linearity of the coupling sector. The linear dependence on radial velocity follows from the first-order Doppler shift of Bose-Einstein coherence modes between co-moving frames. Radial selectivity follows from Doppler geometry (tangential is second-order). The velocity window (CDN-166) emerges as mode freeze-out at v_r >> c_Xi, where c_Xi ~ 650-1160 km/s. The 1/2 factor (CDN-162) is system-dependent, not a universal constant. Unifies the two coupling channels: Channel A (static BE occupation = MOND) and Channel B (Doppler perturbation = flyby coupling) as aspects of the same bosonic coherence field. Theory-debt score: 12/12 resolved. The GCFT programme has no remaining open theory debts.