IPv4 checksum verification and computation (one's-complement, queued with #2470/#2471) #2472

IPv4 checksum verification and computation (one's-complement, queued with #2470/#2471)

Summary

Part of the owner's 2026-10-30 ruling on #2470: the one's-complement checksum work for IPv4/TCP/UDP/ICMP/ICMPv6/OSPF is queued and done together rather than piecemeal, through #2471's shared pseudo-protocol class.

• pcapkit.protocols.internet.ipv4.IPv4.read() does not verify the wire checksum field; a corrupted capture parses silently.
• pcapkit.protocols.internet.ipv4.IPv4.make() leaves checksum unset/zero rather than computing the real checksum (RFC 791 §3.1, header checksum only (no pseudo-header needed for IPv4 itself)).

Scope

Follows the Protocol class conventions, using #2471's pseudo-protocol class for any value from the enclosing IP layer this checksum needs. See docs/source/contributing/pep.rst, "Checksum and Integrity Verification", for the full cross-protocol plan.

Blocked on

#2471 (the shared pseudo-protocol class) landing first.