update

pkg/cart/mutation_change_quantity.go

@@ -26,7 +26,7 @@ import (
 //     (If strict locking is required around every mutation, wrap logic in
 //     an explicit g.mu.Lock()/Unlock(), but current model mirrors prior code.)
 
-func ChangeQuantity(g *CartGrain, m *messages.ChangeQuantity) error {
+func (c *CartMutationContext) ChangeQuantity(g *CartGrain, m *messages.ChangeQuantity) error {
 	if m == nil {
 		return fmt.Errorf("ChangeQuantity: nil payload")
 	}