fix(ipv6): _read_ip_hextet slices include the version nibble

[JarryShaw]

• Searched for similar pull requests
• Followed the coding style (make pylint, make mypy, make isort)
• make test passes, and a test case covers the change
• Added a changelog entry under docs/source/changelog/ and regenerated CHANGELOG.md, if the change is user-visible — N/A, added centrally after the wave

What is the purpose of your pull request?

• fix — corrects a defect

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Description of your pull request and other information

Closes #1094. IPv6._read_ip_hextet read the traffic class from hex[0:2] and the flow label from hex[2:], so both included the version nibble. It now reads them from hex[1:3] and hex[3:]. The parser has used the schema since 45ebc600d, so the helper's only caller is a test, and that test pinned the wrong values. I fixed the method rather than deleting it because it is a method on a public class. The bug was first reported in #389 and was never filed.

Probe (6abcdef0): before (6, 0x6a, 0xbcdef0), after (6, 0xab, 0xcdef0).

Tests: I corrected the pinned values at test_ipv6_unit.py:131. The new module test_ipv6_read_ip_hextet_unit.py checks the issue's bytes, and also checks the helper against the bytes that IPv6.make(...).pack() produces.

• With the fix reverted: new module 2 failed, test_ipv6_unit.py 1 failed, 9 passed.
• With the fix: 2 passed and 10 passed, 5 subtests passed. tests/project: 379 passed, 1 skipped, 1285 subtests passed.
• Style: I ran pylint, mypy and isort on ipv6.py. None of their findings are on the edited lines. Every finding is in code this PR does not change.

[JarryShaw]

Cross-review verdict on e5cdc74ac: GOOD TO GO (reviewed on Sonnet; authored on Opus)

• Fix: the slices now follow RFC 8200: version at nibble 0, traffic class at [1:3], flow label at [3:]. A probe gives 6abcdef0 → (6, 0xab, 0xcdef0), plus three other asymmetric patterns.
• Return shape: unchanged, a 3-tuple of int.
• Independent check: the make().pack() test uses the schema's bit-field packer, not the slicing code under test.
• Fails without the fix: with ipv6.py taken from the base commit, the new module fails 2 and test_ipv6_unit fails 1. With the fix, both pass.
• Callers: there are none outside the two tests.

[github-actions]

Coverage: 88.74% (unit tier, Python 3.14, e5cdc74ac, Unit Tests run success)

Package	Statements	Missed	Branches	Partial	Cover
pcapkit (top level)	104	4	20	4	93.55%
pcapkit/const	18757	1035	2342	846	90.23%
pcapkit/corekit	1874	91	578	22	94.33%
pcapkit/dumpkit	136	0	40	0	100.00%
pcapkit/foundation	2422	143	842	34	92.62%
pcapkit/interface	112	7	40	5	92.11%
pcapkit/protocols	15653	187	3942	162	98.19%
pcapkit/toolkit	487	71	144	3	84.15%
pcapkit/utilities	429	4	122	4	98.55%
pcapkit/vendor	4409	2359	1006	158	42.84%

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