fix(corekit): ProtoChain.__add__ returns stale protocols/aliases

[JarryShaw]

• Searched for similar pull requests

• Followed the coding style (make pylint, make mypy, make isort). Ran isort, pylint and mypy directly on the touched file instead: isort is clean, mypy reports nothing in protochain.py, and pylint's two findings (:57, :194) are on existing lines.

• make test passes, and a test case covers the change (only these modules: the new one, tests/corekit/test_protochain.py and tests/project)

• Added a changelog entry, N/A: added centrally after the wave

• fix — corrects a defect

---

Closes #1099. __add__ used copy.copy(self), which carried the left operand's cached protocols/aliases into the result. It now builds a fresh instance through __new__, as from_list does. __add__ is the only method that copies the chain, and a += b falls back to it.

Probe (a = ProtoChain(Ethernet); a.protocols; b = a + ProtoChain(IPv4)):

• before: b.chain == 'Ethernet:IPv4', but b.protocols == (Ethernet,) and b.aliases == ('Ethernet',)
• after: b.protocols == (Ethernet, IPv4), b.aliases == ('Ethernet', 'IPv4'), and a is unchanged

Tests in tests/corekit/test_protochain_add_stale_cache_1099_unit.py:

• with the fix reverted: 2 failed, 1 passed. The warm-cache and += cases fail. The cold-cache case is a control and passes either way.
• with the fix: 3 passed. test_protochain.py gives 4 passed, and tests/project gives 379 passed, 1 skipped, 1285 subtests passed.

[JarryShaw]

Cross-review verdict on d0a7d7497: GOOD TO GO (reviewed on Sonnet; authored on Opus)

• Fix: the result is now built with __class__.__new__ and __data__, as from_list already does. ProtoChain has no other instance state; its only other attributes are the two cached properties.
• Subclasses: the result keeps the subclass, and the left operand is not mutated.
• Error path: a non-ProtoChain right operand still raises AttributeError, as before.
• Regression test: with protochain.py reverted, the new module gives 2 failed and 1 passed. The warm-cache and += cases fail; the cold-cache control passes.
• Existing tests: test_protochain.py passes (4).
• Other callers: pcapkit has no other + on a ProtoChain and no copy reliance on it.

Nit: a subclass whose __init__ sets extra state would lose it, as it already did under copy.copy. No such subclass exists.

[github-actions]

Coverage: 88.74% (unit tier, Python 3.14, d0a7d7497, 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	1873	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.