Module callback 

Callback operation — external signal coordination.

Implement FR14-FR16: register callback, suspend until signal, handle success/failure/heartbeat signals.

The callback operation is a two-phase operation:

1. DurableContext::create_callback — sends a START checkpoint and returns a CallbackHandle with the server-generated callback_id.
2. DurableContext::callback_result — checks if the callback has been signaled and returns the result or suspends.