docs(protocols): second-round pass over the internet-layer prose (#719)

[JarryShaw]

Please follow the guide below

• Searched for similar pull requests
• Followed the coding style — lint not run; no code changed (AST guard)
• 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?

• docs — documentation only

---

Description of your pull request and other information

Second-round #719 read of all 16 files under pcapkit/protocols/internet/; 7 changed, 9 left as they were. Every change corrects a claim the code contradicts, or past-tense rationale:

• Registry comments (IPv4, HOPOPT, IPv6-Opts, IPv6-Route, HIP, MH ×3): a non-str value is a (parser, constructor) pair, and a missing named method falls back to the unassigned/none/unknown handler. The old text said the value "should be a method".
• HOPOPT / IPv6-Opts Raises:: name schema.len, which is what the code checks. Router Alert said tun and Home Address said jumbo. The threshold line now states the != check.
• HIP: _read_locator returns IPv6Address/LocatorData, not IPv4Address/Locator. PUZZLE and SOLUTION raise when the length is wrong under HIPv1. The docs said they raise on any non-v1 version.
• IPv6-Opts make takes Destination Options, not Hop-by-Hop. "Header Extensive Length" now reads "Header Extension Length". A :class: role that was split across two lines is joined again, which removes one Sphinx -n warning. The IPv6 walk comment is shorter, and its "check above" now reads "this check". That comment still names Shim6 as the code with no dedicated parser that resolves to IPv6_Ext. MH's interval is documented as unused when option is given.

Tests: each tests/protocols/internet/test_*.py was run in its own process, plus tests/test_docstring_contract.py and tests/project. All pass. Sphinx -n, fresh builds, this slice only: 35 warnings on the branch against 36 on main, none new.

File	AST identical to origin/main (docstrings stripped)
hip.py	True
hopopt.py	True
ipv4.py	True
ipv6.py	True
ipv6_opts.py	True
ipv6_route.py	True
mh.py	True

[github-actions]

Coverage: 88.74% (unit tier, Python 3.14, 3f7ba4636, 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.

[JarryShaw]

Cross-review verdict on 3f7ba4636: GOOD TO GO (ran on Sonnet; author Opus, two rounds)

Round 1 (4938f5c6b) was GOOD TO GO with three nits. All three are fixed in this head:

• Shim6: ipv6_ext.py:657 registers it directly to IPv6_Ext, and no dedicated parser exists. Probed: 140 resolves to IPv6_Ext, while 253 and 254 resolve to Raw.
• _make_opt_mesg_id interval: it is read only when neither option nor timestamp is given. option.ntp_timestamp is non-Optional, and its only constructor, at mh.py:3249, always sets it.
• _read_ip_hextet Returns: this now matches the 4-octet (_vers, _tcls, _flow) read.

AST guard: all 7 files are identical to origin/main once bare strings are stripped. test_ipv6_unit, test_mh_unit and test_docstring_contract pass.

UNVERIFIED: the Sphinx -n counts were not rebuilt.