Module Algostream_pairs.Cointegration

Cointegration tests.

Engle_granger is implemented end-to-end. Johansen is wired with the public signature but returns `Not_supported_in_v1 until a linear-algebra dependency is added — Johansen needs an N×N generalized eigenvalue decomposition that we do not roll by hand. The honest stub is intentional: better than silently returning a partial result.