A reduction over an adaptive list (FDA AList.reduce parity). The reduction state is maintained per delta: an insert adds the mapped value, a remove subtracts it (falling back to a full recompute when the reduction cannot invert, e.g. AdaptiveReduction.fold), an update subtracts the old and adds the new. The mirror is aligned with the input positions, so structural ops shift it with the source. Order-sensitive reductions are the user's contract (the reduction's add/sub must be delta-consistent), the same contract as the set/map reduction nodes.