fix(toolkit): the pypcapfile engine silently drops TCP behind an IPv4 AH header #1593

With engine='pypcapfile', TCP carried behind an IPsec AH header over IPv4 (protocol 51) is dropped from TCP reassembly and tracing. No warning is given.

Repro on main (e9f3340f7), 3.11, pypcapfile 0.12.0: Ethernet + IPv4 (protocol 51) + AH (next header 6) + TCP carrying data.

• Default engine: one TCP datagram with payload b'data'. pcapkit/protocols/internet/internet.py:102 dissects AH.
• pypcapfile: no TCP datagram, and no warning.

Cause: _transport (pcapkit/toolkit/pypcapfile.py:459) returns None unless the protocol is 6, and nothing steps over AH. AH over IPv6 is already covered by #1535's warning.

Expected: step over AH, using its Payload Length, to reach the TCP, as the default engine does. Failing that, warn once per capture.

Found by the #1575 worker and its cross-review; I reproduced it independently. Blocked until #1575 merges.