ci: validate changelog fragment filenames (#4586)

* ci: validate changelog fragment filenames

Extend the existing fragment-existence check so it also rejects fragments
whose basename doesn't match <PR_NUMBER>.<type> (where type is one of
added, changed, deprecated, removed, fixed). This catches typos in the
PR number or in the fragment type that would otherwise render a wrong
PR link, or be silently dropped, in the rendered changelog.

Also note in the do-not-edit comment of each towncrier-managed CHANGELOG.md
(root and the 4 independent packages) that the existing static "## Unreleased"
entries pre-date towncrier and must be folded into the first towncrier-built
release manually — towncrier inserts new release blocks above that section
without merging.

Assisted-by: Claude Opus 4.7 (1M context)

* Update CHANGELOG.md

* Update pyproject.toml

* Update CHANGELOG.md

* Update CHANGELOG.md

* Update CHANGELOG.md

* Update CHANGELOG.md

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>