parent item mutations

cmd/cart/promotions_evaluate.go

@@ -86,13 +86,17 @@ func newPromotionEvaluateWithCartHandler(store *promotions.Store, svc *promotion
 			// main.go uses. Going through the shared
 			// PromotionService.EvaluateAndApply means the bypass
 			// loop, the re-eval trigger, and ApplyResults stay
-			// in lockstep with post-add behavior.
+			// in lockstep with post-add behavior. EvaluateAndApply
+			// populates g.EvaluatedItems via cart.MapEvaluatedItems
+			// on its final step, so the preview response reuses the
+			// same per-line projection the live cart renders — math
+			// and rounding stay in lockstep across both paths.
 			svc.EvaluateAndApply(store.Snapshot(), g, previewContextOptions(req)...)
 			resp = promotions.EvaluateResponse{
 				TotalPrice:        g.TotalPrice,
 				TotalDiscount:     g.TotalDiscount,
 				AppliedPromotions: g.AppliedPromotions,
-				Items:             promotions.MapEvaluatedItems(g),
+				Items:             g.EvaluatedItems,
 			}
 		} else {
 			// No live cart (missing/invalid cookie, fetch failed, or