docs(protocols): second-round pass over the link, misc, transport and root protocol prose (#719)

[JarryShaw]

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Second-round #719 pass over pcapkit/protocols/{link,misc,transport}/, protocols/__init__.py and protocols/protocol.py. Only prose the code contradicts, timed context, typos and broken references changed; 14 of 31 files touched.

• Wrong claims fixed: Ethernet and VLAN offset tables; TCP table listed an unparsed tcp.flags.ns; PCAP-NG pack_flags/pack_hash rows named the epb_* methods; unquote's default errors is 'replace', not 'strict'; __repr__/__str__ examples; MP_JOIN Raises:; isb_usrdeliv argument; Transport.register's protocol; "values are tuples" registry comments; L2TPv2's "every non-raising __index__" (PCAP Frame/PCAPNG return a frame number).
• Timed context cut: L2TPv2 (version, dispatch comment), C_Tag.id, the PCAP frame-length commit history.
• L2TP module headings brought to Title Case (no references to them anywhere).

Checks: AST guard below; Sphinx -n on fresh output dirs, 136 warnings for these files on branch and on main, none new; tests/project 379 passed; tests/test_docstring_contract.py and 27 slice modules (one per process) pass. Lint not run.

File	AST identical
pcapkit/protocols/link/__init__.py	True
pcapkit/protocols/link/arp.py	True
pcapkit/protocols/link/c_tag.py	True
pcapkit/protocols/link/ethernet.py	True
pcapkit/protocols/link/l2tp.py	True
pcapkit/protocols/link/l2tpv2.py	True
pcapkit/protocols/link/vlan.py	True
pcapkit/protocols/misc/pcap/frame.py	True
pcapkit/protocols/misc/pcap/header.py	True
pcapkit/protocols/misc/pcapng.py	True
pcapkit/protocols/protocol.py	True
pcapkit/protocols/transport/__init__.py	True
pcapkit/protocols/transport/tcp.py	True
pcapkit/protocols/transport/transport.py	True

[JarryShaw]

Cross-review verdict on fe8731c30: GOOD TO GO (ran on Sonnet; author Opus)

• AST guard: all 14 files are identical to origin/main once bare strings are stripped.
• Offsets: the Ethernet src/type offsets (octets 6/12) and the VLAN TCI/type offsets (octets 0/2) match the field widths.
• TCP:
  • Probed with NS set: flags has no ns key, so dropping that row is right.
  • The _read_mptcp_join Raises: condition matches the code.
  • The ADD_ADDR "IPv4" fix is correct.
• protocol.py:
  • unquote defaults to replace.
  • The new repr(Frame) shape matches a probe on in.pcap.
  • _read_unpack now cites struct.unpack.
• __index__: Frame.__index__() returns its number. PCAP-NG returns _fnum only for packet blocks and raises otherwise, which the "every non-raising" qualifier covers.
• Registries and options:
  • __proto__ values are ModuleDescriptors.
  • register accepts a descriptor or a class.
  • The pack_flags/pack_hash methods exist.
  • C_Tag.id() returns (C_Tag, VLAN).
• Tests: test_docstring_contract, test_tcp_mptcp_join_flag_ordering_unit and test_link_unit pass.

Nit, not raised by this PR: _check_block_floor says a 1–3 octet remainder is caught there, but the floor is < 12. Out of scope here.

UNVERIFIED: no Sphinx run, and no standalone PCAP-NG __index__ probe (that behaviour was read from the code).

[github-actions]

Coverage: 88.74% (unit tier, Python 3.14, fe8731c30, 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.