Cooper.RefCommon (Cooper v0.1.0)

Shared across Cooper.Actions and Cooper.InterpActions: the :ref_suffix (:default/:+alt/:?"msg", CASC.md §7.2's "one rule, not three near-identical ones", reused across @{}/${}/%{} in both grammars) and :env_bracket (${NAME[i]} vs ${NAME[]}) handling, plus the small eval helpers every custom handle_rule in either module needs.