DCRP-38: Covariance-Determinant Rigidity and Low-Rank Vorticity-Phase Collapse

Corrects the claim that 'persistent phase alignment must be non-generic,' derives the fixed core-vorticity covariance matrix equation, proves that periodic full-rank covariance requires a nonzero non-affine residual, and proves that the exact zero-residual periodic branch has rank at most 2.