Computes formal covariance (H^T·H)^{-1} via LAPACK dpotrf_/dpotri_
after DC convergence. Returns upper-triangle array (21 elements for
6-state, 28 for 7-state with B*), condition number from SVD, and
nstate count. Covariance is computed even for perfect-seed fits.
Bumps extension to v0.5.0 with full install SQL and migration path.