Excel Formula Doctor — fix #REF!/#VALUE!/#N/A and broken formulas

POST {"input": "<broken formula or error>"} — returns root-cause diagnosis, corrected modern-Excel formula in a code block, brief explanation of each fix, comma/semicolon locale variants. Handles #REF!, #VALUE!, #N/A, #DIV/0!, #NAME?, circular references. Under 250 words, markdown. Built by a spreadsheet-template factory that validates formulas headlessly in production.