fix(toolkit): the pypcapfile engine drops TCP behind an IPv6 extension header carried in IPv4 #1597

With engine='pypcapfile', TCP carried behind an IPv6 extension header placed directly in IPv4 is dropped from TCP reassembly, and no warning is given. This covers protocol 0 (Hop-by-Hop Options) and protocol 60 (Destination Options).

Repro, using Python 3.11 and pypcapfile 0.12.0. Each capture is Ethernet, then IPv4 with protocol 0 or 60, then a single extension header (next header 6, Hdr Ext Len 0, PadN 4), then TCP carrying data. Results are TCP datagrams reassembled, default engine vs pypcapfile:

The default engine dissects the extension header and finds the TCP inside. pypcapfile drops it.

Cause: _transport in pcapkit/toolkit/pypcapfile.py steps over AH only after the #1590 fix, and over nothing before it. No branch handles a Hop-by-Hop, Destination Options or Routing header inside IPv4.

Expected: step over the extension-header chain the way the default engine does, then treat what follows as TCP. Failing that, warn once per capture.

The #1590 worker found this, and I reproduced it on the three trees above. It is being folded into the stacked #1590–#1593/#1596 branch, because the fix uses the same header walk.