jalapenojson: fixed-width rows for data

Field notes

The header records columns as name:type:size (for example, #scoville:i:7); shorter fields end in a NUL break byte, and sizes count UTF-8 bytes rather than characters. The guide distinguishes offset-based reads for fixed-width rows from row-by-row handling when nested lists are present. A browser visualization lets visitors inspect field positions with hover or keyboard Tab.