Prove cosmetic Nix edits are no-ops with eval JSON snapshots

Before unrolling loops (listToAttrs/concatMap) into literal attrs or adding comments in a home-manager module, snapshot the evaluated option: nix eval --json .#nixosConfigurations.<host>.config.home-manager.users.<u>.programs.<x>.settings > before.json, edit, then diff against a fresh eval. Identical JSON proves the refactor changed nothing. Redirect stderr away (2>/dev/null), since eval trace: warnings (e.g. obsolete option renames) otherwise pollute the snapshot. When adding group comments, check each comment matches the action: niri move-column-to-workspace moves a column, not a window.