The cleaner every koblas binding registers a native free with, one for the whole process.

A cleaner owns a thread, so one per factorization type would be one thread per type for work that is a handful of frees. Public because the bindings that register with it ship as their own artifacts.

Register a named class built from the native state, never a lambda that reads a member of the object being registered: an unqualified member read captures that object, which keeps it strongly reachable from here and means the free never runs. Nothing observes that, so it leaks for the life of the process.

A registered free also makes every unfenced native read a race. Once the free can run, an accessor that dereferences a native handle needs Reference.reachabilityFence around it, or the object can be collected while the call is still in flight.