Function is_in_math_block 

pub fn is_in_math_block(content: &str, byte_pos: usize) -> bool

Check if a byte position is within a $$ ... $$ display-math block.

A block opens only on a $$ that begins its line (see [math_block_ranges]) and closes on the next $$ anywhere, so the closing fence ends the block even when it shares its line with LaTeX content (e.g. \end{cases}$$) or trailing Markdown prose; bytes after the closing $$ are not math.