Function phase_span 

pub fn phase_span(
    run_id: &str,
    plan_id: &str,
    phase: &str,
    variant_id: Option<&str>,
    fold_id: Option<&str>,
) -> Span

Build the per-phase-scope span (ADR-12). run_id/plan_id correlate concurrent or overlapping runs; empty variant_id/fold_id mean the field is not applicable to the current phase. Fields are identifiers only.