feat(core): bridge SshSession recv/send to call protocol via alknet-control:0 channel

Implement the SSH session to call protocol bridge:
- Add FrameFramedReader/FrameFramedWriter for async I/O of length-prefixed EventEnvelope frames
- SshSession::recv() reads InterfaceEvent frames from the alknet-control:0 channel via mpsc
- SshSession::send() writes EventEnvelope frames to the alknet-control:0 channel via mpsc
- Add ControlChannelBridge implementing ControlChannelHandler for routing channel data
- SshHandler::channel_open_direct_tcpip routes alknet-control:0 to the bridge task
- Session Identity attached to every InterfaceEvent produced by recv()
- ControlChannelRouter gains take_handler() for non-control alknet-* channel routing

crates/alknet-core/src/server/control_channel.rs

@@ -60,6 +60,10 @@ impl ControlChannelRouter {
             )),
         }
     }
+
+    pub fn take_handler(&mut self) -> Option<Box<dyn ControlChannelHandler>> {
+        self.handler.take()
+    }
 }
 
 #[cfg(test)]