refactor(corekit): move the stream proxy classes into corekit.io (#1516)

[JarryShaw]

Please follow the guide below

• Searched for similar pull requests
• Followed the coding style (make pylint, make mypy, make isort)
• make test passes, and a test case covers the change
• Added a changelog entry — 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?

• refactor — changes neither behaviour nor performance

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Description of your pull request and other information

_PeekableStream (foundation/extraction.py) and _NamedStream (foundation/engines/pypcapfile.py) move to pcapkit.corekit.io as the public PeekableStream and NamedStream, beside SeekableReader, and join its __all__. Class bodies are unchanged; only the names and one cross-reference differ. Both importers take them from corekit.io, and the io docs page documents them.

• Tests: tests/corekit/test_io_stream_proxies_1516_unit.py pins peek semantics, attribute forwarding, .name and read forwarding from the new module. Against 6af87f608: 15 failed (ImportError / __all__); now 15 passed. The old _NamedStream test in test_pypcapfile_engine.py moves there.
• Selections run, not the whole suite: tests/corekit, tests/foundation/test_extraction*.py, tests/project, tests/test_docstring_contract.py, tests/test_tier_guard.py; all pass.
• No other class moved: every remaining candidate either stays where it is or touches an in-flight file (fix(engines): write real PCAP traces for dpkt and scapy (#1507) #1511, fix(corekit): keep a NumberField's bits outside bit_length as captured (#1488) #1497, the refactor(corekit)!: Info.to_dict() returns an OrderedMultiDict; add MultiInfo and OrderedMultiInfo to infoclass #1484 area). Those moves, and the shared helper functions of refactor: move shared helper functions into utilities or corekit by use case #1519, are left to follow-up PRs.

Closes #1516

[github-actions]

Coverage: 90.08% (unit tier, Python 3.14, 0f6dce8aa, Unit Tests run success)

Package	Statements	Missed	Branches	Partial	Cover
pcapkit (top level)	104	4	20	4	93.55%
pcapkit/const	18795	843	2356	756	92.08%
pcapkit/corekit	2177	66	700	33	96.28%
pcapkit/dumpkit	242	2	88	2	98.79%
pcapkit/foundation	2650	98	934	42	94.98%
pcapkit/interface	112	7	40	5	92.11%
pcapkit/protocols	17086	238	4628	190	97.98%
pcapkit/toolkit	539	74	168	3	84.58%
pcapkit/utilities	431	4	124	4	98.56%
pcapkit/vendor	4409	2342	1006	157	43.25%

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

[JarryShaw]

Verdict: GOOD TO GO at 5aff8a5aa. Cross-reviewed on Sonnet at bf6796e55; the author ran Opus. I then made a pure rebase onto 5f4da2301 (range-diff =, patch-id c32f86965bbd); 79 passed on the rebased tree.

• Both class bodies are AST-identical to the old ones after normalising names and docstrings. Only the class names and one docstring cross-reference changed.
• BytesIO extraction gives identical frames and digests on the default, dpkt and pypcapfile engines, and the fix(extraction): accept a file object without a name as fin (#1506) #1509 scapy refusal is unchanged. The reviewer also ran the real pypcapfile load_savefile against NamedStream.
• Fail-before is 15 on main. On top of that, 11 of 12 code mutations made the new tests fail; an existing test catches the twelfth.
• There is no import cycle: corekit.io imports only utilities. The docs page builds.

Your call, not blocking: pcapkit.corekit re-exports SeekableReader, but not the two new classes. Re-exporting them would be consistent with that; leaving pcapkit.corekit.io as their only address matches EnumLookup, ContextRegistry and others.

Pre-existing defect, to be filed separately: copy.copy and pickling a _PeekableStream both raise RecursionError on main. Measured. __getattr__ reads self._stream before it is set.

[JarryShaw]

Verdict carried to b1ab653cd: GOOD TO GO. This is a pure rebase onto main ffca64637, after #1508 merged: the range-diff is = and the patch-id is unchanged at c32f86965bbd. On the rebased tree, the PR's tests plus #1508's engine-agreement module gave 48 passed, 828 skipped and 433 subtests.

[JarryShaw]

Verdict carried to 0f6dce8aa: GOOD TO GO. This is a pure rebase onto main 81efa6035, after #1525 merged; the range-diff is = and the patch-id is unchanged at c32f86965bbd. main moved only in the HTTP files, which this PR does not touch, and the PR's tests pass on the rebased tree: 79 passed.