fix(hip): HIP_CIPHER warns at six Cipher IDs although RFC 7401 allows six

[JarryShaw]

• Searched for similar pull requests
• Followed the coding style (make pylint, make mypy, make isort) — Makefile flags run on the touched files; mypy and isort clean, pylint adds nothing on the changed lines
• make test passes, and a test case covers the change — modules run: new test_hip_cipher_limit_unit.py, test_hip_unit.py, test_option_roundtrip_unit.py, tests/project
• Added a changelog entry — N/A, added centrally after the wave

What is the purpose of your pull request?

• fix — corrects a defect

---

Closes #1092. RFC 7401 §5.2.8 allows up to six Cipher IDs, and a recipient keeps the first six and drops the rest. _read_param_hip_cipher now warns only above six and truncates to six. length is still the whole wire record, so parsing bookkeeping is unchanged. The maker had no limit and is left alone.

IDs	before (warn / kept)	after
5	none / 5	none / 5
6	warn / 6	none / 6
7	warn / 7	warn / 6

test_hip_unit.py pinned the old warning at six; it now uses seven. Without the fix the new module gives 2 failed, 1 passed (6 and 7 fail, 5 passes); with it, 2 passed. test_hip_unit 34 passed, round-trip 6 passed, tests/project 379 passed, 1 skipped.

[JarryShaw]

Cross-review verdict on 883350174: GOOD TO GO (ran on Sonnet; author Opus)

• Fix confirmed on main. main warns at 6 Cipher IDs and keeps 7, 8 and 9. With the PR, 5–6 IDs parse silently, and 7 or more warn and keep [1..6].
• RFC. The comment matches RFC 7401 §5.2.8 word for word (fetched).
• No desync after truncation. The walk uses the schema len. A following TRANSACTION_PACING parameter still parses (min_ta=10), and frame sizes are right: 72 and 64 octets.
• Fails without the fix. With hip.py reverted, the new module fails 2. The edited test_hip_unit case (7 IDs, len=14) is consistent, at 34 passed.
• By design: a parsed 7-ID parameter now re-makes as 6 IDs (64 octets), so it no longer round-trips byte-identically. That is the RFC's "dropping the rest". 5–6 IDs still round-trip.

[github-actions]

Coverage: 88.74% (unit tier, Python 3.14, 883350174, 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	15655	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.