CDN-345: The Concentration Coherence Test - Art Edition

CDN-345 interpolates between CDN-344’s independent concentration-draw population model and its coherent single-c shift using an equicorrelated Gaussian common-mode parameter rho. The test preserves the CDN-343 marginal prior, c0=10. 0 and sigmaₗog10 (c) =0. 15 dex, while asking how much shared concentration coherence is needed to make SLACS Channel B population closure ordinary. For BE, P (20/20) >=10% is reached at rho=0. 270, but P (20/20) >=20% requires rho=0. 769 and P (>=19/20) >=50% requires rho=0. 779. Verdict: COHERENCETHRESHOLDSPLIT.