Phase C scope decision: Diagnostic carries no class key, so the F1
diagnostic route would need a class field threaded through every
diagnostic site (broad change) for marginal value — tier-2 already
surfaces diagnostics and many duplicate the runtime error classes. Defer
the route + the ~33 diagnostic.* tier-3 blocks to issue #38. v1 ships
command-form hints + 9 runtime error-class hints (comprehensive for
those). Updates ADR-0053 D2/D6/Status/OOS + README.