[Copilot]

In manual mode, XPub.XSend unconditionally set m_lastPipe = null after every completed send. Since m_lastPipe is also used to track the pipe of the most-recently-received subscription (so Subscribe()/Unsubscribe() can target it), any Send() call between ReceiveFrameBytes() and Subscribe() silently dropped the subscription.

Changes

• XPub.cs — XSend: Only clear m_lastPipe when m_lastPipeIsBroadcast is true. The field is needed for broadcast exclusion, not for general sends; leaving it intact for regular publishes preserves the pipe reference that manual-mode Subscribe()/Unsubscribe() depend on.

• XPubSubTests.cs — ManualSubscribeAfterSend: New test covering the broken sequence:

// Subscriber sends subscription → publisher receives it (m_lastPipe = sub_pipe)
var subscription = pub.ReceiveFrameBytes();

// A send between receive and subscribe previously wiped m_lastPipe
pub.SendFrame("A");                              // was: m_lastPipe = null ← bug

// Subscribe must still target the correct pipe
pub.Subscribe(subscription.Skip(1).ToArray());  // was: silently no-op

pub.SendFrame("A");
Assert.Equal("A", sub.ReceiveFrameString());    // was: test would hang