ctypes FFI to cactus_engine.h. The binding module lives at
/python/cactus/bindings/cactus.py.
It loads libcactus_engine.{so,dylib} from /python/cactus/bindings/lib/ (bundled)
or cactus-engine/build/ (dev build), populated by cactus build --python.
For installation, the high-level API, the CLI, and the server,
see /python/README.md.