v6.15.1

Fixed

• The cell type of a pre-calculated formula result – a text result was written without t="str", so Excel treated it as a number and showed #VALUE!; boolean and error results are now written as t="b"/t="e" too (#139)
• A formula with a null pre-calculated result (['=A1+B1', null]) – the cell was written empty and the formula was lost; now it is written without a cached value, so Excel calculates it on open
• Writer prevented garbage collection – it registered [$this, 'removeFiles'] as a shutdown function, which held a global reference to every instance, so the destructor never ran and memory grew in long-running/CLI processes (#138)