Changelog fragments

Each user-facing change ships as one file in this directory instead of editing CHANGELOG.md directly, so concurrent pull requests never conflict.

• Filename: <PR number>.<type>.md, where <type> is one of added, changed, deprecated, removed, fixed, security (the Keep a Changelog section the entry belongs in). A second fragment for the same PR and type gets a counter suffix: 1234.fixed.2.md. Add the fragment after opening the PR, when its number is known.
• Content: the entry as a single line without a bullet or PR link, e.g. Fixed the thing. Towncrier adds the link from the filename. Prefix **Breaking:** for changes that bump the major version.

To release version X.Y.Z on YYYY-MM-DD:

1. Run uv run towncrier build --yes --version X.Y.Z --date YYYY-MM-DD.
2. Add [X.Y.Z]: https://github.com/wkentaro/labelme/compare/v<previous>...vX.Y.Z to the link list at the bottom of CHANGELOG.md.
3. Commit the updated changelog and deleted fragments, then tag that commit.

Prerelease tags render pending fragments without changing files.