Editor: Vega-Lite expression intelligence; single-home render errors

docs/architecture/01-state-and-stores.md

@@ -236,6 +236,13 @@ feature owns — which modal is open, the runtime theme, transient render flags.
 > that's the signal to extract a feature store. A bloated central store is the
 > thing this split exists to prevent.
 
+> Rule: a store earns its place by **decoupling** producers from consumers — a fact
+> belongs in one when more than one component reads it, or when many sites produce it for
+> one surface to consume (the imperative `notify()` / `confirm()` overlay stores). When a
+> single component is both the only producer and the only consumer, the fact is that
+> component's **local `useState`**, not a store — a store there decouples nothing, and is
+> the shape to fold back.
+
 ---
 
 ## 4. Actions: Mutations Live in the Store, Not Components