feat(spec)!: PROTOCOL_VERSION 17 → 18 in an ordinary PR — regenerated spec-changes.json and upgrade guide, ^18 handshakes, pre-mode lockstep exception (#22085 Q1 → B)

feat(spec)!: PROTOCOL_VERSION 17 → 18 in an ordinary PR — regenerated spec-changes.json and upgrade guide, ^18 handshakes, pre-mode lockstep exception (#22085 Q1 → B) #31964