saveTermAndVotedFor

Atomically persists term and votedFor in a single durable write.

Called at every term-advance site (become-candidate, step-down). Implementations MUST write both values atomically — a crash between two separate writes allows a node to vote twice in the same term (Raft §5.1 / §5.2 election safety).

Persistent implementations (SQLite, IndexedDB) MUST implement this as a single transaction: UPDATE raft_meta SET term=?, voted_for=?. Avoid NSUserDefaults on iOS without explicit synchronize() — it is not crash-safe by default.

Note: appendEntries after truncateFrom is a liveness concern on crash (extra round-trip) but not a safety concern — no composite method is required for that pair.