docs(protocols): second pass over HIP, IPv4, IPv6 and IPX (#1103)

[JarryShaw]

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• 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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Summary

#1103 round 2: the files phase 1 left unread, plus the phase-1 reviewers' follow-ups.

Reads what phase 1 left unread: HIP, its schema and data model, the
IPv4 makers from TR on, and the IPv4 schema and data model.

• test_docstring_contract: drop the two ipv4.py pins this fixes, point
  the Vendor.context reason at default.py:417, and say none is excused.
• internet: drop the beta-only EXTENSION_HEADER_LIMIT versionremoved.
• HIP data model: versionremoved for SolutionParameter.lifetime (SOLUTION's Reserved octet is written as a PUZZLE Lifetime, and lifetime=0 escapes a bare ValueError from math.log2 #654).
• HIP: **kwargs on 11 makers, Raises completed on six methods, RFC
  cites for OVERLAY_TTL and VIA_RVS, "schama" typo throughout.
• HIP schema and data: parameter names, two typos, the locator and
  host identity selector cases, the TF list item widths.
• IPv4 schema: the QS function lookup admits 0 to 15, not 0 to 8.
• IPv6: the fix(ipv6): parse unrecognised extension headers generically so the chain survives #891 bullet names both paths to IPv6_Ext, and
  generic_ext_codes is rendered with its IPv6_Ext link resolved.
• from_data, from_dict and data references fixed.

AST identical bar the three permitted test entries; tests pass.

Refs #1103

[JarryShaw]

Verdict: GOOD TO GO at 4ca60c009. Sonnet review and re-check (author: Opus). The first review passed. A fix round then reverted the register Raises: rewording to Protocol (#1016), fixed the IPv6_Ext xref at ipv6.py:115, and made the KNOWN_SWALLOWED comment accurate. The AST guard is identical against the base, tests/project passes, and every added directive says 1.5.0.