Function detect_delimiter_from_path 

pub fn detect_delimiter_from_path(path: &str) -> u8

Pick a default delimiter from a path’s extension.

.tsv → tab, .psv → pipe; everything else (including stdin -) → comma. Case-insensitive. This is only the auto-detect layer — explicit overrides from the CLI flag, READ_CSV 2nd arg, or WEB CTE DELIMITER win over this.