Mark 0.3.1 in the changelog (#631)

Author: TheGreatAxios

CHANGELOG.md

@@ -11,7 +11,7 @@ matching `## [X.Y.Z]` section (plus install instructions). Do not maintain
 parallel copies under `docs/` or `scripts/notes/`. At cut time: rename
 `## [Unreleased]` to `## [X.Y.Z] - YYYY-MM-DD`, then run the release script.
 
-## [Unreleased]
+## [0.3.1] - 2026-08-24
 
 ### Fixed