fix(tcp): MPTCP reserved bits are written back as zero (MP_JOIN, DSS, REMOVE_ADDR, MP_PRIO, MP_FASTCLOSE) #1212

Describe the bug
The MPTCP subtype schemas leave their reserved bits unmapped and the data models have no field for them (schema/transport/tcp.py around :885-888, :925-929, :942-952, :1019, :1037 and :1071-1083).

To Reproduce
Each of these rebuilds with the reserved bits cleared:

• MP_JOIN-SYN: 1e0c1e01… → 1e0c1001…
• DSS: 1e082e01… → 1e082001…
• REMOVE_ADDR: 1e044f01 → 1e044001
• MP_PRIO: 1e035e00 → 1e035000
• MP_FASTCLOSE: 1e0c7fff… → 1e0c7000…

Expected behavior
The reserved bits round-trip unchanged.

Found by the transport round-trip audit (#1202). The repro is from the audit report and I have not re-run it yet.