Excelize: Unbounded <col max> attribute is loaded with no MaxColumns check and expanded per-column by flatCols(), so any column mutator hangs or OOMs the process

Summary The max attribute of a element in xl/worksheets/sheetN.xml is read verbatim on open with no check against MaxColumns, and flatCols then walks Min..Max doing a deepcopy.Copy and append per iteration. Executed: max="300000", only about 18x past the real 16384 limit, cost 46.2 s of CPU and 1...