checkpoint: reshape facade to Stores.Persistent / Stores.Ephemeral()

The Open facade now exposes the two stores under lifecycle-named accessors: Stores.Primary -> Stores.Persistent (field) Stores.Temporary() -> Stores.Ephemeral() (method) and the strategy helpers follow: getCheckpointStore -> getPersistentStore getTemporaryStore -> getEphemeralStore

Callers read sto.Persistent.Read(...) / sto.Ephemeral().Write(...), the symmetric surface from the review feedback. No behavior change.