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, node profile, capability limits).
Source the immutable config and identity reads across the receive hot
path, the XX handshake/session/rekey state machines, and the discovery,
tree, bloom, retry, and lifecycle modules through the context accessors.
Includes the bloom delta/full/NACK/resize and byte-total counters in the
registry. The Node fields and the context are rebuilt in lockstep at every
mutation site.

src/node/tests/session.rs

@@ -1468,8 +1468,9 @@ fn test_purge_idle_sessions_cleans_pending_packets() {
 
 #[test]
 fn test_purge_idle_sessions_disabled_when_zero() {
-    let mut node = make_node();
-    node.config.node.session.idle_timeout_secs = 0;
+    let mut config = Config::new();
+    config.node.session.idle_timeout_secs = 0;
+    let mut node = make_node_with(config);
 
     let remote = Identity::generate();
     let remote_addr = *remote.node_addr();