Release v0.7.0

[katosh]

Summary

Prepare v0.7.0 release. All 41 commits since v0.6.3 are on main; this branch adds release prep on top:

• Set CHANGELOG.md release date to 2026-04-13
• Add missing CHANGELOG entries: Python 3.9 drop (breaking change), DA field_mapping fix
• Apply ruff formatting to 4 files that drifted

Pre-release checklist

• Version 0.7.0 in pyproject.toml
• python_requires >= 3.10
• mellon >= 1.7.1 dependency
• pip check clean
• ruff format --check clean
• python -m build succeeds (sdist + wheel)
• CHANGELOG complete with date
• All deprecated APIs have DeprecationWarning
• All public APIs have docstrings
• No TODOs/FIXMEs/debug code in source
• CI green on this branch

After merge

git tag -a v0.7.0 -m "Release v0.7.0"
git push origin v0.7.0
gh release create v0.7.0 --title "v0.7.0" --notes-file CHANGELOG_SECTION.md