fix(reassembly): IPv4 reassembly leaves a TSO datagram (total_length 0) incomplete #1555

Describe the bug
IPv4 reassembly turns every TSO datagram (total_length 0) into an incomplete one with no payload, under every engine.

Reproduction
Take examples/captures/tcp.pcap with the IPv4 frames' total_length set to 0, and run extract(..., ipv4=True, reassembly=True) on #1554's head (821aa1003):

• default engine: 4 × PARTIAL, payload None;
• dpkt engine: 4 × PARTIAL, payload None;
• unmodified capture: 4 × COMPLETE, payloads of 44, 32, 244 and 32 octets.

I ran this myself.

Expected behavior
The same COMPLETE datagrams as the unmodified capture. A datagram with total_length 0 and DF clear is unfragmented, and its payload is the rest of the frame (#1547).

System information
Linux, CPython 3.14.7, #1554 at 821aa1003.

Additional context
The adapters pass tl=ipv4.len, which is 0 (toolkit/pcap.py:70, toolkit/dpkt.py:537, toolkit/scapy.py:256, toolkit/pypcapfile.py:492). foundation/reassembly/ip.py:155 then computes length = TL - IHL = -20, and :195 sets TDL from it. info.len must stay 0 so the round trip holds, so the fix belongs in reassembly or the adapters.

Found by the #1554 author. This issue is blocked until #1554 merges, because the TSO payload it relies on is new there.