Function lower_set_arg 

fn lower_set_arg(
    target: Target,
    arg: &SetArg,
    attributes: &mut AttrMap,
    metadata: &mut DocumentMetadata,
    current_text_size_pt: &mut f64,
    diagnostics: &mut Vec<Diagnostic>,
)

Convert one parser-level SetArg into an attribute on the Raw node representing this #set directive. Emits semantic diagnostics (MOS0015 unknown key, MOS0020 type mismatch, MOS0027 sanity floor) and updates metadata / current_text_size_pt as a side effect.