Function is_reference_label 

fn is_reference_label(label: &str) -> bool

Whether label can be spelled as an @ reference: i.e. it is drawn from the reference grammar’s alphabet [A-Za-z0-9_:.-] (mirrors scan_label_chars in mos-parse). #figure(label: …) and #image(label: …) accept arbitrary strings, so the label index can hold names such as "intro x" or non-ASCII labels that an @… reference can never name; suggesting one would produce a fix that does not parse.