tcp: MP_CAPABLE parses only 12 and 20 of RFC 8684's five lengths

[JarryShaw]

TCP rejects three of the five MP_CAPABLE forms that RFC 8684 §3.1 defines. Measured on main:

Length	Form (RFC 8684 §3.1)	Result
4	SYN, v1, no key	ProtocolError: TCP: [OptNo 30] invalid format
12	SYN/ACK, sender's key	parses
20	ACK, both keys	parses
22	first data ACK: both keys + Data-Level Length	ProtocolError
24	first data ACK: both keys + Data-Level Length + Checksum	ProtocolError

Cause: pcapkit/protocols/transport/tcp.py:1550 raises unless schema.length in (12, 20). The comment above it cites §3.1 but names only those two lengths. The schema matches that limit: MPTCPCapable.rkey is ConditionalField(UInt64Field(), lambda pkt: pkt['length'] == 20) (schema/transport/tcp.py:894-897), skey is unconditional, and there is no Data-Level Length or Checksum field. Figure 4 makes the sender's key conditional on "Length > 4" and the receiver's key on "Length > 12".

Fix:

• Accept lengths 4, 12, 20, 22 and 24.
• Make skey conditional on length > 4 and rkey on length > 12.
• Add the Data-Level Length (16 bits, present from length 22) and the optional Checksum (16 bits, length 24).
• Mirror all of this on the make side.
• Add a parse and a round-trip test for each of the five lengths.

Blocked on #1034 and #1040, the open docstring-only PRs for the #719 sweep that edit transport/tcp.py and schema/transport/tcp.py. Starts once both merge or close. Found while reviewing #1040.

[JarryShaw]

Unblocked: #1034 and #1040 merged. A worker is on the fix now.