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 sigma_log10(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: COHERENCE_THRESHOLD_SPLIT.