fix(corekit): an untyped ListField packs the bytes it unpacks (#1489)

[JarryShaw]

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Without an item_type, ListField.unpack returns the field's octets as one bytes, but ListField.pack iterated that value and rejected each int, so pack(unpack(b)) raised FieldValueError. ListField.pack now writes such a bytes value as-is (pcapkit/corekit/fields/collections.py:145-146). A typed ListField still packs item by item, and an untyped list of bytes items is still joined. ListField.__init__ is untouched, and a trial merge with #1493 is clean.

• Removes the Gap(1489, ...) row from tests/corekit/test_field_roundtrip_unit.py.
• New tests/corekit/test_listfield_untyped_pack_1489_unit.py: 4 of its 9 checks fail on main's collections.py, and all pass with the fix.
• Ran sequentially rather than make test: tests/corekit 474 passed; tests/protocols/test_capture_roundtrip_runtime.py 4 passed; tests/project 419 passed, 1 skipped; tests/test_docstring_contract.py 7 passed.
• repr(frame.info.to_dict()) hashed per file over the 23 files in examples/captures is identical with and without the fix.

Closes #1489

[JarryShaw]

Verdict: GOOD TO GO at e220f17f5. Cross-reviewed on Sonnet; the author ran on Opus.

• The fix works. I measured it myself: on origin/main sources the new test file gives 4 failed, 5 passed. At head, that file plus test_field_roundtrip_unit.py gives 14 passed, 739 subtests, and only the Gap(1489…) row is removed.
• The shortcut skips nothing. It fires only for an untyped field given bytes. OptionField and ChunkListField unpack to lists, so their schema packing is unchanged. The per-capture to_dict() hashes are identical across all 23 captures.
• It merges cleanly with fix(corekit): give unnamed ListField, PayloadField and SchemaField a default name (#1491) #1493, which edits ListField.__init__ in the same file (git merge-tree).
• Tests: tests/corekit 474 passed; the SCTP tests 71 passed; tests/project 419 passed.

Non-blocking:

• Like a typed ListField, an untyped one neither pads nor truncates bytes to length, which differs from BytesField. The round trip is unaffected.
• bytearray and memoryview still take the item path.

[github-actions]

Coverage: 89.65% (unit tier, Python 3.14, e220f17f5, Unit Tests run success)

Package	Statements	Missed	Branches	Partial	Cover
pcapkit (top level)	104	4	20	4	93.55%
pcapkit/const	18795	958	2356	871	90.99%
pcapkit/corekit	2109	68	674	29	96.16%
pcapkit/dumpkit	242	2	88	2	98.79%
pcapkit/foundation	2648	98	930	48	94.80%
pcapkit/interface	112	7	40	5	92.11%
pcapkit/protocols	17086	238	4628	190	97.98%
pcapkit/toolkit	539	74	168	3	84.58%
pcapkit/utilities	431	4	124	4	98.56%
pcapkit/vendor	4409	2342	1006	157	43.25%

Per-file detail: the coverage-html artifact of this run.