Finalize published artifacts with digest, signature, SBOM, and provenance verification

[litroc]

Context

Implements LI-125, MLX-100, and REL-30.

Scope

Pull the real published collection and container artifacts by immutable reference and verify versions, acceptance profile, signatures, SBOM, provenance, and digests.

Acceptance criteria

• Finalizer validates real published artifacts rather than only build output.
• Collection and container versions match the accepted candidate.
• Signature, SBOM, provenance, manifest, and platform digests verify.
• Tampered signature or digest mismatch blocks completion.
• Mutable convenience tags move only after finalizer success.
• Final evidence is linked here and to LI-125.