chore(repo): back-merge v0.2.21 release commits into dev

[w7-mgfcode]

After cutting v0.2.21 (PR #364 → #365), release-please bumped .release-please-manifest.json, pyproject.toml, and CHANGELOG.md on main (commit 2712e49). dev still reads 0.2.20, and main is 3 commits ahead.

This issue tracks the main → dev back-merge so main becomes an ancestor of dev and future dev → main release PRs stop re-conflicting.

• Merge the back-merge PR with --merge (a real merge commit), never squash — otherwise main never becomes a dev ancestor.

[w7-mgfcode]

Done in #367 — back-merged main (v0.2.21) into dev. dev now reads 0.2.21 and main is an ancestor of dev. (Auto-close didn't fire because #367 targeted dev, not the default branch.)