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Shared support for the exact-region developments

The modules under Copula.Rank.Region.Common (namespace ProbabilityTheory.Copula.RankRegion.Common) collect the auxiliary results that the XiRho, XiBlest, XiBeta, TauFootruleBeta and MeanVariance region developments share: centered ordinal sums with a countermonotonic block, diagonal-band and two-strip copulas, finite increasing shuffles, conditional-copula constructions, stochastic monotonicity bounds relating ξ, ρ and the footrule, ramp and tent integrals, measurable functional witnesses, and continuity along mixtures.

The historical module paths Copula.Rank.Region.<X>.Support.* (and Copula.Rank.Region.XiBeta.{TwoStrip,TentIntegral}) remain as thin compatibility modules that export these declarations into their original namespaces, so every old name denotes the same constant as its Common counterpart.