Opaque capability token for ActorRef<MeshAdminAgent> across the
Python boundary. No methods, no getters — Python never inspects
this. It exists solely to transport the typed ref from
_spawn_admin to _start_periodic_snapshots_http.
The Python-bootstrapped client root: the root client instance plus the
client proc mesh registered by bootstrap_host. Discovered by
python_client_root for callers that need the stable Python root.