A by-name REFERENCE input edge: the node READS the target as context but is NOT
recomputed when the target changes. Still a real input (validated, ordered by
depth so the target settles first, read at recompute) — it just doesn't
propagate. For a node whose recompute is expensive/non-deterministic and
consults mutable reference data it shouldn't be re-triggered by (an LLM step
that looks up a human-curated people/positions table). Contrast ref, which
dirties this node on change.