Function is_in_inline_math 

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

Check if a byte position is within inline math ($...$).

Only single-$ spans count here. A $$...$$ token is display-math syntax, and whether it is actually math depends solely on whether it begins its line - that decision belongs to [math_block_ranges]. The regex still consumes $$...$$ tokens so a single-$ span cannot straddle them, but a mid-line $$...$$ is a literal here, not inline math, keeping this function consistent with the line-start-gated block model.