Function locate_label 

pub(crate) fn locate_label(
    src: &str,
    start: usize,
    end: usize,
) -> Option<(ParsedLabel, usize)>

Locate the first <label> token in src[start..end], returning the parsed label and the byte offset just past its >. Unlike strip_trailing_label this finds a label anywhere in the range, so the heading parser can detect a label that is not the trailing element (e.g. = Title <id> trailing) and flag it (MOS0048) instead of silently swallowing it into the text.