Release-note fragments

Each PR adds a small Markdown fragment here. Towncrier builds the changelog at release time; do not edit CHANGELOG.md for pending changes.

From the development environment:

just change +csv-units.fixed.md   # opens the editor; choose a unique slug
just changes                    # preview without writing or consuming files

Use 123.added.md when an issue/PR number is already known, or +unique-slug.added.md otherwise. Write a short sentence describing the change for users, without a leading bullet. Filenames accept lowercase letters, digits, and hyphens in slugs. Use ordinary Markdown; avoid relative file links because the same note appears in the repository, site, and GitHub release.

Types: breaking, security, removed, added, changed, fixed, documentation, or development. For a change that needs no public release note, add an internal fragment explaining why; those explanations are checked but excluded from rendered release notes.

just check validates all filenames and nonempty content. PR CI also checks for a new fragment; release PRs that update the assembled changelog are exempt. The site builds an upcoming-changes preview directly from these files. Releases use just release and just release-pr; see the release workflow.