Neither Python example was registered as a test, so `ctest -L examples`
in CI skipped them entirely -- which is how 08's missing Python node
went unnoticed.
Add a kpn_python_example() helper (gated on KPN_BUILD_PYTHON) that runs
each script with PYTHONPATH pointed at the freshly-built module, so it
does not depend on cwd or a hard-coded build/python path, and register
07 and 08. Also del the network in 07 for deterministic teardown.
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.
