fix(corekit): a cut list of plain values reads the items past the capture as zeros #2760

Describe the bug
A cut list of plain values reads the items past the capture as zeros. #2758 stopped this for lists of schema items, but the plain-value branch of ListField._unpack_items still reads on through the zero-fill. Every plain-value ListField is affected (IGMPv3 sources, IPv6-Route ip, HIP, MH, SCTP, IPv4, TCP, Radiotap), framed and standalone alike.

Reproduction

from pcapkit.protocols.internet.igmpv3.protocol import IGMPv3
rec = bytes([1,0,0,3, 224,0,0,1, 10,0,0,1, 10,0,0,2, 10,0,0,3])
rep = bytes([0x22,0,0,0,0,0,0,1]) + rec
[str(s) for s in IGMPv3(rep[:20]).info.records[0].sources]  # ['10.0.0.1', '0.0.0.0', '0.0.0.0']

Expected behavior
['10.0.0.1']: the list ends with the data, and only the item the cut lands inside reads its missing octets as zeros. This is the rule #2758 wrote into ListField.unpack. The rebuild must stay byte-exact (it is today).

System information
Linux, CPython 3.14.10, pcapkit 1.5.1 at c211805ae.

Additional context
Fix site: the plain-value branch of ListField._unpack_items, mirroring the schema branch. Makers that derive a count or length from the items (e.g. IPv6_Route._make_hdr_ext_len) must keep the declared value, as HTTPv2._make_http_settings does after #2758.