Function distinct_arranged_errs 

pub(crate) fn distinct_arranged_errs<'a, T: RenderTimestamp>(
    errs: Arranged<'a, TraceAgent<Spine<ArcBatch<OrdKeyBatch<MzStack<((DataflowErrorSer, ()), T, Diff)>>>>>>,
    name: &str,
) -> Arranged<'a, TraceAgent<Spine<ArcBatch<OrdKeyBatch<MzStack<((DataflowErrorSer, ()), T, Diff)>>>>>>

Rewrites an arranged error collection to hold each of its errors once.

Sound because query semantics depend only on whether an error is present, and correct under retraction only because it reads the accumulated collection: no pointwise function of the input diffs (a saturating add, a sign) can collapse multiplicity and still cancel when the errors retract.

NOTE: One consumer does read the multiplicity. Error-count introspection reports it as the number of failing rows, so log_dataflow_errors must see the collection before this collapses it, or a dataflow reports one error however many rows failed. Collapse after the logging, never before.