Attaching ECN on send erases the socket's DSCP #5

Problem

ecn::build writes the bare ECN bits as the IP_TOS / IPV6_TCLASS control
message. The kernel uses that value as the whole traffic-class byte for the
datagram (ip_cmsg_send: ipc->tos = val), which overrides whatever DSCP the
socket was given with setsockopt(IP_TOS).

Repro: sender with setsockopt(IP_TOS, 0xb8) (EF), control ecn = ect0. The raw
TOS on the wire is 0x02, not 0xba.

Proposed fix

Pick one:

• Carry DSCP alongside ECN in the transmit control, for example a traffic_class
  feature whose value holds both fields, with ecn as a convenience on top.
• Let a socket option record the DSCP at open time, and have ecn::build OR it
  in. This needs somewhere to keep it, which the stateless feature design does
  not have today.
• At minimum, document on ecn and in docs/metadata.md that sending with it
  resets DSCP to zero.

Acceptance

A test that sets DSCP and sends with ECN observes both on the wire.