scrollback does not survive muxd upgrade (v1 gap)

open   by a73x

muxd upgrade v1 hands over grid state + pty fds but drops engine history. Reattach after an upgrade starts with an empty scrollback. Deliberate v1 cut (user-approved 2026-08-19): history is more bytes and a bigger serialization surface. Fix = extend the handover payload with history pages, versioned, gated by the same xversion handover leg.