Function prune_index_version 

fn prune_index_version(
    current: &SeriesIndexVersion,
    visible: &HashSet<FileId>,
    expired_index_ids: &[FileId],
    stats: &mut ReconcileStats,
) -> Option<SeriesIndexVersion>

Prunes obsolete metadata without building indexes or retiring published handles. Returns None without cloning the current version when no entries need removal. Physical range deletion belongs to the SST purger; series handles are retired only after the cleaned catalogs and snapshot have been published successfully.