Fix actions/checkout from non-existent v6 to v4.3.1 with SHA pin

The @v6 tag does not exist for actions/checkout; current stable is v4.
Pin to the immutable commit SHA for v4.3.1 to prevent tag-squatting
and ensure reproducible builds.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>