ci: validate changelog fragment filenames (#5212)

* ci: validate changelog fragment filenames

Add a workflow step that rejects fragments whose basename doesn't match
<PR_NUMBER>.<type> (where type is one of added, changed, deprecated,
removed, fixed). This catches typos in either the PR number or the
fragment type before they produce a misleading PR link in the rendered
changelog. towncrier check verifies a fragment exists; it doesn't
enforce naming.

Also note in the do-not-edit comment of the root CHANGELOG.md that the
existing static "## Unreleased" entries 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

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