The tunnel bind conns (bindConn/bindUDPConn) now expose their metadata (the
tunneled hostname) via the xctx.Context interface, so the service accept loop
forwards it into the handler context. handleRawForwarding selects a hop node by
that hostname when a hop is configured, so a node with a matching filter.host is
picked; otherwise it falls back to dialing the hostname directly. Fixes
reverse-proxy-tunnel TCP routing (gost#898) where the docs example failed with
"node not available" because the mux path never set the host metadata.
