CDN-346: The Mean-Coherence Tradeoff

CDN-346 maps the tradeoff between median halo concentration and common-mode coherence in the GCFT/CDN lensing closure framework. Using the locked CDN-344/345 prior model with sigma_log10(c)=0.15 dex, it shows that the high-coherence demand found at fixed c0=10 is not unique: a sub-one-sigma upward shift in median concentration can substitute for much of the required coherence. The locked result is ONE_SIGMA_MEAN_SHIFT_CLOSES_INDEPENDENT_TAIL.