The example was named "python subport" but its graph was entirely C++
(ProduceNode -> DoubleItNode); Python only tapped the output, leaving a
dangling "#todo: return value to network".
Rewrite so the only node in the graph is a pure-Python py_triple, driven
from both ends via the subport taps: net.write() injects inputs and
net.read() pulls results back, closing the round trip. Also del the
network at the end so its callable cycle is reclaimed deterministically.
