mutation fixes

pkg/cart/cart-mutation-helper.go

@@ -73,6 +73,33 @@ func (c *CartMutationContext) ReleaseItem(ctx context.Context, cartId CartId, sk
 	return c.reservationService.Release(ctx, inventory.SKU(sku), l, inventory.CartID(cartId.String()))
 }
 
+// equalOptional is the canonical nullable-equality primitive used by the
+// cart mutations when deciding whether two identifier fields (parent/child
+// links, store ids, anything else nullable) refer to the same thing.
+//
+// Returns true when both pointers are nil, when both alias the same
+// address, or when both are non-nil and dereference to equal values. The
+// same-address short-circuit is a no-op for correctness but lets a caller
+// who passes the same variable get back true without traversing the
+// generic's comparison path.
+//
+// One tested primitive covers every *T the cart currently compares —
+// `*uint32` for ParentId, `*string` for StoreId — and any future
+// comparable pointer type. Hand-rolled "both nil OR both non-nil and
+// equal" expressions were duplicated at AddItem's merge site and were a
+// footgun whenever a new field was added (which axis of the predicate
+// was the relevant one?). Keeping a single helper makes future merges
+// safer to copy.
+func equalOptional[T comparable](a, b *T) bool {
+	if a == b { // both nil OR same address
+		return true
+	}
+	if a == nil || b == nil {
+		return false
+	}
+	return *a == *b
+}
+
 // CascadeChildQuantities scales every direct child's quantity
 // proportionally when a parent's quantity is bumped (ChangeQuantity or
 // the AddItem-merge path).