Function merge 

pub fn merge(entries: Vec<BufferEntry>) -> MergeResult

K-way merges every buffer entry by buffered_at — this is the relative order WITHIN one merge operation only. It never looks at what is already committed in the database: whatever comes out as clean still has to go through apply_clean_entries, which stages each entry through the table’s normal put/patch/delete path and so always lands after the current end of the log, no matter how early buffered_at claims to be — this is what satisfies the “never retroactive” rule, not anything in this function.