fix: verify published release attestations

[soerennb]

Summary

• read the current GitHub artifact-attestation API envelope
• decode the DSSE payload and verify the published archive digest
• verify that the attestation was produced by the tagged release workflow

Verification

• the exact verification logic passes against the published v0.10.0 tarball, checksum, SBOM, and attestation
• two independent public-history sanitizer runs produced the same commit
• gitleaks history scans passed and no internal metadata is present

This changes only post-publish verification logic; the already published release artifact is unchanged.