fix(protocols): a later IPv4 fragment's data is dissected as the upper-layer header #1553

Describe the bug
A later IPv4 fragment, one with a non-zero Fragment Offset, has its payload dissected as if it began with the upper-layer header. The payload is really mid-datagram data. This is the IPv4 counterpart of #1545.

Reproduction
IPv4(bytes(IPv4(protocol='UDP', offset=185, payload=Raw(bytes(range(40))), src='1.1.1.1', dst='2.2.2.2'))) re-parses as IPv4:UDP:Raw, with a fragment offset of 1480. The 8 payload octets at the start are read as a UDP header.

Expected behavior
Leave the data of a later fragment as Raw and decode no upper layer, as #1545's fix (PR #1552) does for IPv6 and as Wireshark does. A first fragment, offset 0 with MF set, is still dissected. Reassembly and every round trip are unchanged.

System information
Linux, CPython 3.14.7, main at 16fbea054.

Additional context
pcapkit/protocols/internet/ipv4.py never consults the offset before decoding the next layer. Found by the #1545 author. This is blocked until #1547's PR merges, because that PR is editing the same payload-length path in ipv4.py.