feat(cluster): RFC 010 c6b — handshake on the accept/connect path

Drive the c5 machines as straight-line code on the path (D8): dial_handshake
and accept_handshake do the IO on a shared FramedConn, and a connection actor
is spawned only after a successful handshake. Rejects, tie-break losses (D7),
protocol faults and timeouts are all resolved on the path by closing, so no
actor ever exists for a connection that did not establish. The whole
FramedConn travels into spawn_established, carrying any read-ahead past the
handshake frames.

Handshake deadlines land here rather than in c6c: FramedConn::recv_deadline
enforces them between reads via the connection's fd arm, so a peer that
connects and goes silent cannot wedge the acceptor.

Connection lifetime moves to the manager (pulled forward from c7). The path
registers each established connection and hands over its ConnHandle; the
manager owns it, monitors the actor, and tears the connection down on
Disconnect, on peer close, or at manager shutdown. spawn_established returns
a Pid, so a connection neither outlives nor dies with whichever actor
established it — the ownership that made two-node teardown unorderable.

The manager also tracks in-flight dial intents, monitored so a panicking
dial cannot wedge the tie-break, and answers HelloCtx for the accept path.

src/cluster/transport/tcp.rs

@@ -197,6 +197,10 @@ pub struct TcpListener {
 }
 
 impl Listener for TcpListener {
+    fn readable_arm(&self) -> Option<crate::scheduler::FdArm> {
+        Some(crate::scheduler::FdArm::readable(self.inner.as_raw_fd()))
+    }
+
     fn accept(&mut self) -> io::Result<Box<dyn Conn>> {
         loop {
             wait_readable(self.inner.as_raw_fd())?;