strategy: extract AdvanceCommittedPrimary helper

AdvanceCommittedPrimary sets refs.Primary to hash and best-effort advances refs.Mirror. Replaces (SetReference + MirrorBestEffort) pairs across EnsureMetadataBranch, metadata_reconcile, cleanup, condensation, attribution, finalization, and the post-fetch mirror site. The best-effort mirror wrapper becomes unexported and takes refs explicitly. MirrorCommittedMetadataRef (strict) stays exported for attach and explain summary writes, where surfacing failure is more useful than silent best-effort.