Constant MAX_MARKDOWN_BYTES 

pub const MAX_MARKDOWN_BYTES: u64 = _; // 16_777_216u64

Upper bound on a single Markdown source, shared by every input path.

File and stdin readers both enforce this so an unbounded source (a pipe, a character device such as /dev/zero, or a pathologically large file) cannot exhaust memory. A file’s reported length is not trusted: some sources report zero yet stream without end, so readers cap the byte count directly.