[Artifact cache] Roll out verified reuse and retire baseline-run selection #9534

Goal

Enable fingerprint-based reuse progressively and remove legacy baseline-run selection only after demonstrated parity.

Work

• Begin with a limited, documented set of supported workflows and configurations.
• Keep reuse behind an explicit enable/bypass control during rollout.
• Run comparison jobs that build without artifact reuse.
• Compare correctness, build work, total latency, failures and operating cost.
• Expand coverage only after agreed acceptance gates.
• Retire legacy baseline-run search and copy behavior after sustained parity.
• Retain ordinary source-build fallback after legacy selection is removed.

Release branches, manual runs and other producer/consumer combinations remain explicitly enabled or disabled according to the reviewed policy.

Acceptance criteria

• Supported workflows show correct results and useful measured savings.
• Comparison jobs detect compatibility regressions.
• Bypass and rollback procedures are demonstrated.
• Monitoring and ownership are established.
• Legacy behavior is removed only after the parity gate is accepted.
• Ordinary source-build fallback remains available.
• Documentation reflects the deployed behavior.

Dependencies

Task 10 and sufficient rollout evidence.