Graft

Build-channel injection for carriers with a finished arm — the vocabulary a generic recursion-scheme driver needs to feed a decoration's build seam without knowing the concrete variants.

done injects an already-finished payload: the consumer must treat it as final (an apo graft places it in the result slot as-is; a futu unroll expands the prebuilt layer without consulting the coalgebra again). step injects a focus alongside its leftover context — the keep-going arm.

The payload meaning of done is pinned per optic value via the existential X (Fst[X]), not by this capability — see the Gather/Scatter decoration optics in cats-eo-schemes.