A stable sort node (FDA AList.sortWith parity, poll model). The keys are computed with their input positions (the sortByi mapping contract); the sort is stable by position. The source is re-read on every read (dependency registration); keys, sort and diff run only when the source's version moved, so clean reads allocate nothing and run no user code (library invariant 5). The keyMapping/comparer must be pure functions of the element — external adaptive reads are not tracked.