Function callsite_set 

pub fn callsite_set(
    cell: &'static OnceLock<&'static PropertySet>,
    pairs: &'static [(&'static str, &'static str)],
) -> &'static PropertySet

Interns the literal properties of one macro callsite on first use. cell is a static emitted by the macro, so each callsite pays one atomic load afterwards.

Duplicate names are resolved the way merge_properties does: case-insensitive, last wins.