Function brief 

pub fn brief(
    index: &Index,
    files: &[(CanonVersion, Guidance)],
    from: Option<CanonVersion>,
    destinations: &[String],
    selections: &Selections,
) -> Briefing

Read one interval’s guidance against one target.

A breaking step is a decision the operator accepts with the step’s own body in front of them. An additive step is reported and blocks nothing. Partial coverage is a decision rather than a gap, because the operator can still say they know what is missing.