docs(corekit): fourth whole-repo pass over corekit, const and vendor (#1103)

[JarryShaw]

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• You will be asked some questions, please read them carefully and answer honestly

• Put an x into all the boxes [ ] relevant to your pull request (like that [x])

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• Followed the coding style (make pylint, make mypy, make isort)

• make test passes, and a test case covers the change. Narrow runs only, listed below.

• 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

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Tick the commit type your subject line carries.

• fix — corrects a defect
• feat — adds a feature
• perf — changes performance, not behaviour
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• test — tests only
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• release — bumps the version or rolls up a distribution
• chore — anything else

The fourth whole-repo prose pass of #1103, over corekit, const and vendor. It changes prose only: every changed .py file's AST is unchanged once string expressions are blanked, except where a docstring is added or vendor template text is edited together with its generated const file.

Cross-references:

• point the get overrides hidden by :meta private: (Method, AppType)
  at literal names or the class, so the rendered text keeps the class
• link the per-transport AppType registries by full path in the
  register_alias template and its generated docstring
• use the roles and modules the Python inventory carries: sys.version_info
  is data, SEEK_* live in os, str is a class
• point vendor request() docstrings at their own module's DATA

Accuracy:

• TS flag registry cites RFC 791, not RFC 719
• L2TP Type's request() describes L2TP packet types, not TCP checksums
• drop a stray full stop in an LLA option code description

Counts and wording:

• word vendor comments on IPX, EtherType and AppType without counts
• "cf." for "c.f.", "a collection of", "work like", "modelled on"

Refs #1103

[JarryShaw]

GOOD TO GO at 335c80597. A Sonnet cross-review found no blocking problems. The _EnumRegistry.__missing__ docstring is the only non-prose AST change, and it was expected. Two optional notes are left as they are: "all three services on TCP/80" in the apptype template is an IANA registry fact, and the retargeted vendor DATA refs were unresolved before as well.

[github-actions]

Coverage: 90.07% (unit tier, Python 3.15, 335c80597, Unit Tests run success)

Package	Statements	Missed	Branches	Partial	Cover
pcapkit (top level)	104	2	20	2	96.77%
pcapkit/const	19595	881	2618	867	92.11%
pcapkit/corekit	2504	70	792	36	96.54%
pcapkit/dumpkit	276	3	100	3	98.40%
pcapkit/foundation	2916	164	1016	34	93.59%
pcapkit/interface	138	7	52	5	93.68%
pcapkit/protocols	17411	224	4580	189	98.10%
pcapkit/toolkit	872	177	298	8	77.18%
pcapkit/utilities	522	2	158	4	99.12%
pcapkit/vendor	4431	2341	1018	156	43.64%

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