fix(toolkit): the pypcapfile engine parses a later IPv4 fragment's data as a TCP header #1576

With engine='pypcapfile', TCP reassembly reads a later IPv4 fragment's data as a TCP header, and a bogus segment comes out of it.

Repro on main (cc084357f), Python 3.11, pypcapfile 0.12.0. A PCAP holds a SYN, then an IPv4 fragment with offset 1480, protocol 6, and 40 octets of data starting AAAA.

• Default engine: no TCP datagram.
• pypcapfile: a TCP datagram whose payload is b'B' * 20.
• The fix(toolkit): pypcapfile TCP reads past VLAN tags, warns of tunnels, skips later fragments and padding (#1537, #1576, #1577) #1575 worker's fuzzing hit the same thing on 59 of 5,000 frames, including flows on port 16705 (0x4141) read out of payload bytes.

Cause: _transport() in pcapkit/toolkit/pypcapfile.py only checks the protocol number, not whether the frame is a first fragment. #1562 fixed the default engine's version of this.

Expected: a fragment with a non-zero offset is not parsed as a TCP header, as in the default engine.