Let plugins hold Objo objects opaquely and return them later

Plugins can receive and return only the mapped value types today (numbers, Boolean, String, MemoryBlock, Picture, DateTime, Colour, exported plugin types, and arrays of those). Plugins that embed other runtimes, or act as generic containers, need to accept an arbitrary Objo object, hold it for as long as they like, and hand the same object back later: a script engine storing a host object in a script variable is the clearest case, and any Tag-like object slot wants the same thing.

Requested: an opaque handle type in the SDK (for example ObjoObject) that can appear in exported method signatures in both directions. The handle must preserve identity (the object handed back is the same instance), stay opaque (no member access through the handle), and keep the Objo object alive for as long as native code holds the handle, releasing it automatically under the existing disposal rules.