- detectAllHeaderSets() finds multiple header groups per row (left 1st / right 15th)
- isBlankRowForHeaderSet() checks blanks per column range for dual layouts
- parseSheetRows() scans rows 0-4 for header row, processes each set independently
- analyzeRow() computes due_day from date/label/pattern with fallback to defaultDueDay
- Cell type validation allows 's' (shared formula) type
- Non-numeric amounts (auto, double pay, past due) become detected labels
- Day patterns (1st, 15th, 24th) parsed as due_day values
- Security: bounds validation in isBlankRowForHeaderSet, anchored regex, label sanitization
null