Function generation 

pub fn generation() -> u32

An opaque token that changes on every environment switch.

Record it when deriving state from the environment — an index built over a Store, a map hydrated from one — and compare on access: a different value means the derived state describes an environment that is no longer active and must be rebuilt. This is the same mechanism Store uses to reset itself, exposed for state the stores can’t see. The load is relaxed and costs nothing on hot paths.