mirror metadata refs after DeleteOrphanedCheckpoints

DeleteOrphanedCheckpoints commits a cleanup tree on entire/checkpoints/v1 and advances the local branch, but did not update the v1.1 custom ref. In v1.1 mode the mirror still pointed at the pre-cleanup tip, so every NewCommittedReadStore consumer (list/status/explain/dispatch) kept surfacing the just-deleted checkpoints until another v1 write happened to mirror.

Mirror after the SetReference, gated by checkpoints_version through MirrorCommittedMetadataRefBestEffort.