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/unit.rs

@@ -1003,6 +1003,38 @@ fn active_peer_same_path_discovery_refreshes_stale_peer() {
     ));
 }
 
+#[tokio::test]
+async fn node_context_mirrors_config_and_immutable_facades() {
+    let mut node = make_node();
+
+    // The immutable facades read the shared NodeContext.
+    let expected_addr = *node.identity().node_addr();
+    assert_eq!(node.node_addr(), &expected_addr);
+    assert!(!node.is_leaf_only());
+    let _ = node.uptime();
+    assert_eq!(node.config().peers().len(), 0);
+
+    // update_peers must rebuild the context so config() — which now reads the
+    // context — reflects the new peer list. Guards the copy-on-write sync.
+    let peer = Identity::generate();
+    let new_peer = crate::config::PeerConfig {
+        npub: peer.npub(),
+        alias: None,
+        addresses: vec![],
+        connect_policy: crate::config::ConnectPolicy::OnDemand,
+        auto_reconnect: false,
+        via_nostr: false,
+    };
+    node.update_peers(vec![new_peer]).await.unwrap();
+
+    assert_eq!(
+        node.config().peers().len(),
+        1,
+        "config() must reflect update_peers through the rebuilt context"
+    );
+    assert_eq!(node.config().peers()[0].npub, peer.npub());
+}
+
 #[tokio::test]
 async fn update_peers_races_new_alternative_without_dropping_active_peer() {
     let mut node = make_node();