Type Alias FallbackDecision 

pub type FallbackDecision = (Option<Box<dyn FnOnce(TcpStream) -> Pin<Box<dyn Future<Output = ()> + Send>> + Send>>, bool);

A fallback callback’s decision for one (src, dst) flow: an optional per-connection handler and whether this callback intercepts the flow. Matches Go’s (handler func(net.Conn), intercept bool):

• (_, false) — decline; the manager tries the next registered callback.
• (Some(h), true) — claim the flow; h services the connection.
• (None, true) — claim the flow and reject it (the connection is closed).