Let plugins call back into Objo through callable handles

A plugin cannot call back into Objo code at a time of its own choosing. The only path from a plugin into Objo today is raising a declared event, which delivers marshalled values to subscribed handlers synchronously. Script engines and similar plugins - anything shaped like C++ callbacks: comparators, iterators, promise continuations - need to receive a reference to an Objo method (for example through AddressOf), store it, and invoke it later with primitive arguments, receiving a return value.

Requested: a callable handle type that crosses from Objo into the plugin, with an Invoke that accepts the mapped primitive signatures and returns a mapped value; defined threading (invocation during a plugin call runs synchronously on the VM thread, exactly as event dispatch already does); and defined error propagation when the Objo code throws.