fix(tcp): MPTCP ADD_ADDR in the RFC 8684 layout does not parse #1217

Describe the bug
Only the RFC 6824 IPVer layout is understood. The nibble at schema/transport/tcp.py:994-997 is read as version, and transport/tcp.py:1789 rejects any value other than 4 or 6. There is no HMAC field.

To Reproduce
1e083101 c0000201 (E=1 echo) raises FieldError: TCP: [OptNo 30] 3 invalid IP version. 1e103001 c0000201 0001020304050607 (with an HMAC) fails the same way.

Expected behavior
The RFC 8684 layout (reserved bits, E, address family from the length, optional 8-octet HMAC) parses and rebuilds.

Found by the transport round-trip audit (#1202). I re-ran the repro on main at 630f0a581.