[ahl]

The flattening helpers took a serde seq and rejected everything else at the data-model layer, so a tuple, a fixed-size array, a tuple struct, or a newtype struct around one of these failed to splice even though JSON writes each as an array. The serializer now treats serialize_tuple and serialize_tuple_struct like serialize_seq and forwards a newtype struct to its inner value; the deserializer visits the remaining elements for a tuple or tuple struct and forwards a newtype struct the same way. Variants stay rejected, since their tag is not part of an array.