stream xml from file

Parses an XML file and runs the attached function once per batch of elements named like the element pin, passing pins batch (1-based) and items; each element becomes an object of its attributes and child elements (repeated children become arrays, mixed text lands in _). batch defaults to 100 and is clamped to 1-1000. success=false with error ELEMENT_EMPTY (empty element), FILE_EMPTY (no file), FUNCTION_EMPTY (no function wired) or FILE_DOES_NOT_EXIST. The remove and children pins are declared but not read by the Go executor: the file is never deleted.