docs(protocols): Protocol docstring claims it is the base of all protocol families

[JarryShaw]

• Searched for similar pull requests
• Followed the coding style (make pylint, make mypy, make isort) — not run; docstrings only
• make test passes, and a test case covers the change — ran tests/test_docstring_contract.py and tests/project only; no new test, a docstring change has no failing-without-fix test
• 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
• feat — adds a feature
• perf — changes performance, not behaviour
• refactor — changes neither behaviour nor performance
• test — tests only
• docs — documentation only
• ci — workflows or build tooling
• chore — anything else

---

Description of your pull request and other information

Closes #1101.

Before: Protocol said "Abstract base class for all protocol family", but Protocol.__subclasses__() is []; ProtocolMeta said it supports Protocol and generates attributes for it, but its body is empty and it is ProtocolBase's metaclass.
After: Protocol is the user-facing base for third-party protocols and new protocol stacks; ProtocolBase names the families that derive from it directly (Link, Internet, Transport, Application, pcapkit.protocols.misc) and its metaclass; ProtocolMeta says it adds nothing over abc.ABCMeta. Wording follows docs/source/ext.rst and docs/source/pcapkit/protocols/protocol.rst.

Evidence

• AST guard (bare-string Expr stripped, ast.dump(include_attributes=False)) vs origin/main b10e7951b: identical.
• tests/test_docstring_contract.py: 7 passed, 12 subtests passed
• tests/project: 379 passed, 1 skipped, 1285 subtests passed

[JarryShaw]

Cross-review verdict on 8c278abf8: GOOD TO GO (ran on Sonnet; author Opus)

• AST guard: protocol.py is identical to main once docstrings are stripped.
• Probed:
  • Protocol.__subclasses__() is [].
  • ProtocolBase's direct subclasses are Protocol, the four layer classes, and the five misc classes (Frame, Header, PCAPNG, NoPayload, Raw).
  • ProtocolMeta adds nothing over ABCMeta and is type(ProtocolBase).
• Guidance: what the docstrings say about which class to subclass matches ext.rst and protocol.rst.
• Tests: test_docstring_contract (7) and tests/project (379) pass.

UNVERIFIED: no Sphinx build was run, so the :class: references to abc.ABCMeta were not checked.

[github-actions]

Coverage: 88.74% (unit tier, Python 3.14, 8c278abf8, 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.