node: extract immutable state into a shared context and atomic metric registry

Store node counters in an atomic metric registry read through &self, and
introduce a shared NodeContext bundle holding the effectively-immutable
fields (config, identity, startup epoch, capability limits). Source the
immutable config and identity reads across the receive hot path, the
handshake/session/mmp/encrypted state machines, and the discovery, tree,
bloom, retry, and lifecycle modules through the context accessors rather
than direct field reads. The Node fields and the context are rebuilt in
lockstep at every mutation site.

src/node/tests/forwarding.rs

@@ -757,8 +757,9 @@ fn test_detect_congestion_with_transport_drops() {
 
 #[test]
 fn test_detect_congestion_disabled_ecn() {
-    let mut node = make_node();
-    node.config.node.ecn.enabled = false;
+    let mut config = Config::new();
+    config.node.ecn.enabled = false;
+    let mut node = Node::new(config).unwrap();
 
     // Even with transport drops, disabled ECN should return false
     let tid = TransportId::new(1);