Add descriptor-non-redundancy vs task-non-additivity distinction to halt-head section

README.md

@@ -191,6 +191,14 @@ per-perturbation ordering of E[N] (best |ρ| = 0.23, below a 0.3 novelty ceiling
 The signature is reproducible *within* a cell type but does **not** port across cell
 lines (cross-line ρ = 0.14) — it is a cell-type-specific property.
 
+Two claims must be held apart. As a **descriptor**, E[N] is *non-redundant* with the
+network (above): it captures per-perturbation structure the graph statistics miss. For
+one downstream **target-class ranking task**, however, it is *non-additive* with a
+STRING baseline built from the same functional modules — adding |ΔE[N]| gives no AUC
+lift over response + STRING in three of four lines. Non-redundancy as a descriptor and
+non-additivity for that one classifier are different statements; only the latter is
+negative, and it does not diminish the signature's novelty.
+
 ---
 
 ## Provenance & caveats