TimeoutNowStaleTerm

onTimeoutNow guard 1: the frame's term is below ours — a §3.10 transfer from a leader we have already moved past.

Not redundant with TimeoutNowSenderNotEstablishedLeader, and reachable with no attacker at all. leaderForTerm is by construction a fact about currentTerm, so a stale-term frame from the node's own pinned leader clears that later guard and this is the only thing refusing it: L sends TimeoutNow(T), the frame is delayed, L wins again at T + 1, the follower re-pins L, and the old frame lands. Replayable indefinitely, and each replay would be a pre-vote-less election (#1980).