Stage345 Artifact Download Verification Engine

Decision: accept

Checks:
- github_run_success: OK
- artifact_download_success: OK
- github_anchor_receipt_found: OK
- local_session_hash_match: OK
- artifact_receipt_hash_match: OK
- artifact_run_binding_match: OK
- stage343_accept: OK
- stage344_ready: OK

Meaning:
Stage345 turns GitHub artifact verification readiness into actual artifact download verification.