Function emit_unknown_facts 

pub fn emit_unknown_facts<I>(
    store: &mut FactStore,
    prov: &FactProvenance,
    unknowns: I,
) -> usize
where
    I: IntoIterator<Item = (String, String)>,

Emit one Opacity fact per (target_logical_id, reason) pair — the “unknown” family. reason is the evidence string (typically a stringified UnknownReason) so a consumer can explain why the analyser could not see through the target.