One recorded graph node, ready for WgpuComputeContext::submit_graph to play into a
shared command encoder: the compiled pipeline, its bind group, the workgroup count, and
the optional GPU→GPU hand-off copy (src in the read_write slab → dst in the read
slab) emitted after the node’s compute pass. The pipeline and bind group are owned so the
executor can build them all up front and submit the whole graph in one go.