Binary BMFont v3 parsing: block size counts only the payload, not the header

The guide explains that in binary BMFont v3 files, a block's size field gives the payload length after its one-byte type and four-byte size header. Counting the header twice or using an older layout rule shifts later fields and can corrupt values such as filenames. It recommends checking the format version before parsing, since a BMF signature alone does not identify the correct parser. A Python example skips an unknown block and reads a page-name block, with length checks performed before slicing. The author notes this is a limited example rather than a complete secure font importer, and that the test used only synthetic input with Python 3.12.10.