NativeRingAddress

The address of a kprt_ring the sink owns, as an opaque number.

An address rather than a typed pointer so that no cinterop type reaches this module's public API. That is an ABI decision and NOT a safety one: this was never made safe by its type. It is made safe by RawRingApi, by there being exactly one producer (the sink that created the ring) and exactly one consumer (the engine's ring wrapper), and by the sink owning the ring for its whole life. Misusing a number here is exactly as bad as misusing the pointer was.