v1.2.0 — the systems metric

Adds systems_metric — an exact, auditable, directed cost between two structured shapes: the cheapest admissible transformation path carrying A into B, priced in the tropical (min, +) semiring over the framework's named morphisms. Directed (A->B may differ from B->A), obeys the triangle inequality, and honest-stops with incommensurable=True when no path exists. Additive & backwards-compatible. 13 new tests (suite 338). See CHANGELOG.