Python plugin runtime for Fidius.
This crate embeds CPython into the host process via PyO3 and (in later
tasks) exposes a PluginHandle whose dispatcher calls into a loaded
Python module. Hosts opt into Python plugin support by depending on this
crate (typically through fidius-host's python feature flag).
At this stage the crate provides only the foundation: shared interpreter
initialisation and Python-exception-to-PluginError conversion. The
loader, dispatcher, and packaging integration land in subsequent tasks
under FIDIUS-I-0020.