Function threshold_local 

fn threshold_local<'scope, T: RenderTimestamp>(
    arrangement: Arranged<'scope, TraceAgent<RowRowSpine<T, Diff>>>,
    name: &str,
) -> Arranged<'scope, TraceAgent<RowRowSpine<T, Diff>>>

Thresholds a dataflow-local ok arrangement, keeping rows with a positive count.

The reduce goes through a concrete-spine helper because reduce_abelian’s higher-ranked output-key bound only normalizes when the input and output trace types are concrete through a function signature.