Remove public whole-state save API

[halqme]

Summary

Remove the exported Store.Save(State) API so stale whole-state persistence cannot be reintroduced by future callers.

• initialize a new database through a private initializeState helper
• keep initialization on the existing transactional syncStateTx path
• replace Save-oriented persistence tests with Update + Load round-trip coverage

Why

Normal writes now use Store.Update(expectedRevision, mutate), but leaving Store.Save(State) exported still makes it easy for future code to bypass CAS and overwrite newer state. This PR removes that escape hatch entirely.