CDN-273: The Argument

Derives E/kT = sqrt(g/a0) from harmonic Xi modes in the gravitational potential. The coherence field's effective spring constant is proportional to local gravitational acceleration (kappa_g propto g), giving oscillation frequency omega propto sqrt(g) and mode energy E propto sqrt(g). The proportionality constant cancels exactly in the ratio E/kT, yielding the Bose-Einstein argument sqrt(g/a0) parameter-free. Combined with CDN-272's identity nu = 1 + n_BE(sqrt(x)), this completes the derivation chain from Xi Lagrangian to the McGaugh/MOND interpolation function. D11 FULLY PAID. Only D1 (v_r-linearity) remains open. Theory-debt score: 11/12.