Row-level diff of two CSVs (a→b) — reconcile two exports. With `key` (a column name, or a column index when header:false), rows pair by key → added / removed / changed (per-field old→new). Without a key, a whole-row multiset diff gives added/removed only. Delimiter auto-detected (, ; tab |); tolerant RFC-4180 parsing. First row is the header unless header:false. Max 5 MB per input; reported buckets cap at 10000.