Name the release notes of the next release unreleased.md

[ru551n]

Names the release notes of the next release release_notes/unreleased.md instead of release_notes/0.2.0.md, so the notes don't name a version before it has been decided.

• Releasing: the commit that bumps the version in pyproject.toml renames unreleased.md to release_notes/<version>.md. The release workflow publishes that file, as before.

• Fail early: tools/release.py validate --tag vX.Y.Z fails while release_notes/unreleased.md still exists, so a tag can't publish a release without its notes. Without --tag (the workflow_dispatch rehearsal) it doesn't check, since the file still exists while a release is rehearsed.

  .../release_notes/unreleased.md still exists. Rename it to release_notes/0.2.0.md in the commit bumping the version.
  

• Docs: the release notes page lists unreleased.md first, titled "Unreleased", then the releases, newest first. It no longer compares against the version in pyproject.toml.

• publish.yml: the comment on the release step mentions the rename.

Open PRs

#15, #16, #18, #21 and #26 add entries to release_notes/0.2.0.md. When rebased, those entries move to release_notes/unreleased.md.

Tests

• validate --tag v0.1.0 fails with the message above, and validate without a tag passes.
• The docs build with -W, with "Unreleased" then "0.1.0".
• ruff and the license header check pass.

🤖 Generated with Claude Code