ImplausibleNegativeTerm

The implausible-term bound's malformed arm: the frame's term is negative.

Terms start at 0 and only increase, so a negative one is not a stale term — it is a corrupt or fabricated field. It lands below ours, which is why it never actually reaches the wedge report even though wedgeGate names one: RaftEngine.noteRefusedLeaderFrame counts only frames at or above our own term, and a frame from behind us is the gate working rather than a node that cannot make progress.

A separate guard from ImplausibleTermJump since #1989 (they shared one if), and it must stay upstream of it: the jump test is a subtraction, and it is only free of Long wrap because this arm has already refused a negative left operand.