chore(changelog): regenerate the Unreleased block from changelog.d/

[lusoris]

Summary

make docs-fragments-check — a required check — fails on pristine master.
Merged PRs add fragments under changelog.d/ but nothing re-renders
CHANGELOG.md's Unreleased block, so the two drift apart. The gate names its own
remedy, which is all this PR runs:

scripts/release/concat-changelog-fragments.sh --write

A finding I did not fix here

Regenerating prints:

WARNING: changelog.d/chore/ is not a Keep-a-Changelog section; fragments here are SKIPPED.
WARNING: changelog.d/refactor/ is not a Keep-a-Changelog section; fragments here are SKIPPED.

Those two directories hold 27 fragments — 24 under chore/, 3 under
refactor/ — whose content has never reached CHANGELOG.md.

The generator's section list is the canonical Keep-a-Changelog set
(added changed deprecated removed fixed security), so the skip is arguably by
design: chores and refactors are not user-facing. But the directories exist,
authors keep writing to them, and the only signal is a stderr warning nobody sees
unless they run the script by hand.

That is a policy question with two defensible answers — map them onto Changed,
or delete the directories and reject those sections at PR time — so it is
reported rather than decided unilaterally. Happy to do either.

Type

• build / ci — tooling / infra

Checklist

• Commits follow Conventional Commits.
• make docs-fragments-check exits 0 (it exits 2 on master).
• Unit tests — no source change; CHANGELOG.md is generated output.
• No SIMD/GPU code path touched.
• No new source files.
• Not a breaking change.
• No ADR — mechanical regeneration; the open policy question is flagged above, not settled here.

Bug-status hygiene

• docs/state.md — no runtime bug; this is a generated-artefact sync.

Netflix golden-data gate

• I did not modify any assertAlmostEqual(...) score.

Deep-dive deliverables

• Research digest — no digest needed: trivial.
• Decision matrix — no alternatives: the gate prescribes the command.
• AGENTS.md invariant note — no rebase-sensitive invariants.
• Reproducer / smoke-test command — below.
• CHANGELOG fragment — changelog.d/changed/regen-unreleased-block.md.
• Rebase note — no rebase impact: generated changelog-only sync; CHANGELOG.md and changelog.d/ are fork-added.

Reproducer

git checkout master && make docs-fragments-check     # exit 2
scripts/release/concat-changelog-fragments.sh --write
make docs-fragments-check                            # exit 0

See the dropped fragments:

ls changelog.d/chore/ changelog.d/refactor/ | grep -c '\.md$'   # 27

Known follow-ups

• Decide the chore/ + refactor/ question above.
• This will drift again on the next merge wave. Worth considering whether the
  regeneration should be a release-please step rather than a manual one, since
  the gate is required and the fix is purely mechanical.