A screen or terminal sub-connection was keyed solely by a MasterID that any
TCP peer may supply, so any internet host could claim an online device's
MasterID and bind a sub-conn in its place, hijacking the live screen stream
or a pending shell session. The ConnAuth signature cannot be verified on the
server because its key lives in a vendored client library, so the trust
anchor here is the source IP: sub-conns are always opened device-to-server
from the same host as the login conn. BindScreenConn and BindTerminalConn
now reject a sub-conn whose peer IP differs from the device's recorded
main-connection PeerIP.
Co-Authored-By: deepseek-v4-pro
