Function build_index_version 

async fn build_index_version(
    worker_id: u32,
    store: &ObjectStore,
    region: &MitoRegionRef,
    version: &Arc<Version>,
    requested_bucket_width: Duration,
    now_ms: i64,
    purger: &IndexFilePurger,
    unpublished: &mut UnpublishedSeriesFiles,
    enable_range_index: bool,
    allow_builds: bool,
) -> Result<(Option<SeriesIndexVersion>, ReconcileStats)>

Builds a changed snapshot, retaining reusable indexes and removing obsolete coverage. Returns None when no range or series indexes were added or removed.