Build a deep learning network that sums single digits from the MNIST dataset; the summation must be performed by the network itself.
Implement build_digit_sum_network(input_shape) using only the Python standard library. Return a deterministic architecture specification for two grayscale MNIST images per sample. The specification must use one shared digit encoder, combine the two encoded representations, and produce one scalar output per sample without an explicit arithmetic addition operation.
input_shape is [batch_size, 2, 1, 28, 28]. Return a JSON-compatible dictionary containing the input shape, output shape, shared-encoder flag, explicit-addition flag, maximum supported target, and ordered layer names.
def build_digit_sum_network(input_shape):