Function group_files_into_series_buckets 

pub(crate) fn group_files_into_series_buckets(
    files: &[FileHandle],
    width_secs: i64,
    compaction_window_secs: i64,
) -> Vec<SeriesBucket>

Groups SSTs across levels into sorted, disjoint time buckets without planning builds.

Both widths must be positive, and width_secs must be a multiple of the compaction window width. Inclusive SST ranges are rounded outward to aligned, half-open intervals in seconds. Overlapping intervals merge; adjacent ones stay separate. Each merged bucket tracks its maximum sequence and any unknown sequence.