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.
