fix(internet): a cut behind an IPv4 Total Length of 0 reads checksums INVALID #2689

Found by the short-read audit, and confirmed by an independent re-probe. Neither case is a regression; the first is the #2653 symptom on the TSO path, which #2668 left out.

• IPv4 with Total Length 0 (TSO), cut by snaplen. IPv4.read reads a Total Length of 0 as "the rest of the frame", so the frame's lengths are the only record of the cut. Internet._decode_next_layer clears frame_length regardless, so GRE, ICMP and IGMP behind such a packet read their checksum INVALID and raise a false ChecksumWarning. The NOTE in _decode_next_layer admits the cut is lost.
• GRE(cut) constructed directly, with the header cut inside the checksum or an earlier field. It verifies the zero-filled checksum and reads INVALID. It never consults the header's __short_read__, which it already has.

Fix:

• _decode_next_layer keeps the cut when the layer declared no length. The re-probe validated a declared=False path that the shared verifier carries to ICMP and IGMP, and that AH passes on.
• GRE.read reports TRUNCATED whenever its header carries a short-read record.
• Tests cover each protocol behind a TSO IPv4 at every cut, and a GRE header cut directly.