Function is_nested 

pub fn is_nested(files: &[Vec<String>]) -> bool

Whether every file’s columns are contained in the widest file’s: the shape schema evolution produces, read cleanly as a union. No score or threshold: a file either brings a column no other has or not (overlap ratios cannot tell drift from unrelated tables sharing a key). Failing is not a refusal: the row goes inside, and (all files) still opens the union. Fewer than two files, or files without columns, are one table.