CSV data quality checker

The checker reports, it never edits: rows are scored against your rules and failures are located, but the data itself is left exactly as pasted. File line numbers come from a quote-aware scan, so a quoted field holding a line break still points at the true line. Null cells are skipped by every rule except required, which keeps one missing cell from failing three rules at once. The per-rule score and the headline pass rate answer different questions: a column can score 90% while the file holds far fewer fully clean rows. Everything runs in this tab; nothing is uploaded.