docs(changelog): fourth whole-repo pass over the changelog (#1103)

[JarryShaw]

Please follow the guide below

• You will be asked some questions, please read them carefully and answer honestly

• Put an x into all the boxes [ ] relevant to your pull request (like that [x])

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• 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. Narrow runs only, listed below.

• 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

What is the purpose of your pull request?

Tick the commit type your subject line carries.

• fix — corrects a defect
• feat — adds a feature
• perf — changes performance, not behaviour
• refactor — changes neither behaviour nor performance
• test — tests only
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• ci — workflows or build tooling
• release — bumps the version or rolls up a distribution
• chore — anything else

The fourth whole-repo prose pass of #1103, over the changelog. It changes prose only: every changed .py file's AST is unchanged once string expressions are blanked, except where a docstring is added or vendor template text is edited together with its generated const file.

Missing entries:

• add 1.5.0 entries for the library, docs-build, conda and vendor-gate
  changes merged after the last recorded one, the LLACode and NTPTimestamp
  breaking changes marked as the file marks them; extend the citation range

Accuracy:

• the register entry no longer says get's fallback logs CRITICAL, and the
  sentinel entry names the exported Sentinel base
• correct above/below pointers, a garbled from_data rewrite, doubled
  citations and a stray semicolon; note 1.4.1's TCP(srcport=N) behaviour
• 1.4.0 records the HOPOPT, IPv6-Opts and HTTP/2 fixes of its TCP change

Polish and rendering:

• owner, not maintainer; misspelled, not misspelt; no "since fix(corekit): pad a short field read on the tail, not the head (#604) #621 merged"
• rewrap ragged short source lines, words unchanged
• changelog.rst's notes render as admonitions, not a blockquote
• regenerate CHANGELOG.md

Refs #1103

[JarryShaw]

NEEDS CHANGES at 82a571207. docs/source/changelog/1.5.0.rst:348-349 now says the call returns default "quietly since" #1845. That is timed context, and the parentheses are nested. Suggested fix: say it returns default and logs nothing, and cite #1845/#1873 alongside the other refs. Then regenerate CHANGELOG.md with util/changelog_md.py.

[JarryShaw]

GOOD TO GO at a409479cc. A Sonnet re-review probed get on a miss with a default and found it returns the default with an empty log, which matches the new wording. util/changelog_md.py --check passes, and the commit touches only that entry and its CHANGELOG.md line. The timed-context objection does not apply to this file, because #719 exempts changelog entries from that rule; the rewrite still removes the nested parentheses.