A poll node that rebuilds its output from the source and emits the positional diff (the gap-sheet poll-node strategy for rev, sort, pairwise). The source is re-read on every read. Rebuild-on-every-read is deliberate: the build function may read additional adaptive inputs (the subA/takeA/skipA bounds do), so a source-version gate would be unsound — a change to an external input would be missed. Nodes whose output is a pure function of the source only (e.g. the dedicated SubListNode) use the gated pattern instead, holding invariant 5 on clean reads.