feat(cluster): RFC 010 c6a — connection actor + manager subtree

Per-peer connection actor as a single select-loop plain actor owning the
whole FramedConn: one select folds its command inbox and the transport's
readable arm, so reads and control share one execution context — no reader
thread, no read/write split. The handshake is bypassed here (c6b wires it);
the actor is spawned already-established and self-registers with the manager.

Manager gen_server: the peer-name -> conn-pid registry and the uniqueness
source the handshake's NameTaken depends on. It monitors each connection, so
the table self-heals on any exit path. Explicit supervision subtree keeps the
manager up; connections are dynamic and monitored, never restarted (c7
re-dials).

Transport gains an additive Conn::readable_arm -> Option<FdArm> (default None;
TCP returns its fd's arm, loopback stays None). Existing c3 transport tests
unchanged.

Lifecycle test over localhost TCP: up reflected in the table, commanded
shutdown reaps exactly one, peer EOF reaps the other.

src/cluster/transport/tcp.rs

@@ -180,6 +180,10 @@ impl Conn for TcpConn {
             Err(_) => "<disconnected>".to_string(),
         }
     }
+
+    fn readable_arm(&self) -> Option<crate::scheduler::FdArm> {
+        Some(crate::scheduler::FdArm::readable(self.fd()))
+    }
 }
 
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