node: extract lifecycle supervisor FSM, migrate substrate fields (behavior-neutral)

Introduce a sans-IO lifecycle supervisor: a synchronous step(event) ->
[action] state machine (SupervisorFsm) that authors the substrate-child
spawn and teardown order, plus an owner struct (Supervisor) holding the
substrate runtime fields and embedding the FSM. The substrate-lifecycle
fields leave Node's flat list into the owner - state, packet_tx, the TUN
reader/writer handles + shutdown fd + channels, the DNS task + identity
channel, the Nostr/LAN rendezvous drivers, and the encrypt/decrypt worker
pools; the dataplane keeps packet_rx.

start()/stop() become the driver executing the FSM's SpawnChild/StopChild
actions: same children, same order, same warn/debug-and-continue on
optional failures, same logs, same NodeState transitions. Behavior- and
wire-neutral. The only determinism change is that transports now tear
down in ascending-id order, previously nondeterministic HashMap iteration.

The bounded Draining phase and the Running{Full|Degraded} health split
land as separate follow-on commits.

src/node/tests/handshake.rs

@@ -285,8 +285,8 @@ async fn test_run_rx_loop_handshake() {
     node_b.packet_rx = Some(packet_rx_b);
 
     // Set node state to Running (transports need to be operational)
-    node_a.state = NodeState::Running;
-    node_b.state = NodeState::Running;
+    node_a.supervisor.state = NodeState::Running;
+    node_b.supervisor.state = NodeState::Running;
 
     // === Phase 1: Node A initiates handshake to Node B ===