/// # Scope cap
///
/// Real callback round-trips require the caller to register a GenServer PID
/// that implements `handle_info/2` to handle
/// `{:trait_call, method, args_json, reply_id}` messages and then calls
/// `complete_trait_call` or `fail_trait_call` with the reply.
/// Gleam emits the registration and reply shims here; wiring the callback
/// module is done via the Elixir/Rustler side (existing GenServer pattern).
