saveTermAndVotedFor_persistsBoth

Verifies the §5.1/§5.2 atomicity contract: after RaftStorage.saveTermAndVotedFor both term and votedFor are visible together. Persistent implementations must write both in a single transaction so a mid-write crash cannot leave a node with an advanced term but stale vote (or vice-versa), which would allow it to vote twice in the same term.