docs(tests): tighten the foundation and corekit test prose from the post-sweep merges (#719)

[JarryShaw]

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• Followed the coding style (make pylint, make mypy, make isort) — not run; comment-only change, AST guard below

• make test passes, and a test case covers the change — the four touched modules and tests/project, one process each

• Added a changelog entry under docs/source/changelog/ and regenerated CHANGELOG.md, if the change is user-visible — N/A — centralised in docs(changelog): shared 1.5.0 changelog — long-lived, merges last (#610, #616, #617, #618, #620) #657

What is the purpose of your pull request?

• docs — documentation only

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Description of your pull request and other information

Final #719 pass over tests/foundation/** and tests/corekit/** prose added by the commits since ef0964c81. Comments only.

• Drops "used to" / "no longer" history from the tests: stop re-importing pcapkit before every test (purge_modules dominates suite time) #1065 comments in test_extraction.py and test_fields_collections.py, keeping the reason (the class shares one import).
• test_fields_field.py: the ledger comment blamed "a sibling class, another module"; each class purges first, so its field module (and _zero_pad_ledger) is its own. The leak is from earlier tests of the same class.
• test_foundation.py: setUp does not swap in a fresh import for every test, it puts the class's own one back.

File	AST equal
tests/corekit/test_fields_collections.py	True
tests/corekit/test_fields_field.py	True
tests/foundation/registry/test_foundation.py	True
tests/foundation/test_extraction.py	True

[JarryShaw]

Cross-review verdict on a4d12fbff: GOOD TO GO (ran on Sonnet; author Opus)

• No code changed: the AST of all 4 files is identical to efc9c3774 once bare strings are stripped.
• The three reworded claims hold against tests/_support.reimport_once_per_class:
  • the class-shared ledger
  • "puts the class's own import in sys.modules"
  • per-class tables restored
• The deleted "sibling class, another module" clause had become false after tests: stop re-importing pcapkit before every test (purge_modules dominates suite time) #1065.
• Tests pass: the 4 touched modules, the same 4 run together in one process (76 passed), and tests/project (375 passed, 1 skipped).
• UNVERIFIED: no actual pytest-xdist run.

[github-actions]

Coverage: 88.74% (unit tier, Python 3.14, a4d12fbff, 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.