Function build_looped_layer 

fn build_looped_layer(
    plans: &[StemPlan],
    timeline: &IndexMap<i64, i64>,
    total_ms: i64,
    level_db: f64,
    spec: &LoopSpec,
    render: bool,
) -> Result<(Segment, Vec<Label>)>

Shared loop for the ambience and vintage-filter layers. With render off it computes only the labels (compute_*_labels), without decoding.