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/discovery.rs

@@ -1110,8 +1110,8 @@ async fn test_check_pending_lookups_default_sequence_unreachable() {
     node.pending_lookups
         .insert(target_addr, PendingLookup::new(0));
 
-    let baseline_initiated = node.stats().discovery.req_initiated;
-    let baseline_timed_out = node.stats().discovery.resp_timed_out;
+    let baseline_initiated = node.metrics().discovery.req_initiated.get();
+    let baseline_timed_out = node.metrics().discovery.resp_timed_out.get();
 
     // --- t = 1100ms: first retry deadline (1*1000) ---
     node.check_pending_lookups(1100).await;
@@ -1124,7 +1124,7 @@ async fn test_check_pending_lookups_default_sequence_unreachable() {
         assert_eq!(entry.last_sent_ms, 1100);
     }
     assert_eq!(
-        node.stats().discovery.req_initiated,
+        node.metrics().discovery.req_initiated.get(),
         baseline_initiated + 1,
         "retry #1 must invoke initiate_lookup exactly once"
     );
@@ -1140,7 +1140,7 @@ async fn test_check_pending_lookups_default_sequence_unreachable() {
         assert_eq!(entry.last_sent_ms, 3100);
     }
     assert_eq!(
-        node.stats().discovery.req_initiated,
+        node.metrics().discovery.req_initiated.get(),
         baseline_initiated + 2,
         "retry #2 must invoke initiate_lookup exactly once more"
     );
@@ -1156,7 +1156,7 @@ async fn test_check_pending_lookups_default_sequence_unreachable() {
         assert_eq!(entry.last_sent_ms, 7100);
     }
     assert_eq!(
-        node.stats().discovery.req_initiated,
+        node.metrics().discovery.req_initiated.get(),
         baseline_initiated + 3,
         "retry #3 must invoke initiate_lookup exactly once more"
     );
@@ -1168,12 +1168,12 @@ async fn test_check_pending_lookups_default_sequence_unreachable() {
         "8s window not yet expired: pending_lookup must persist"
     );
     assert_eq!(
-        node.stats().discovery.req_initiated,
+        node.metrics().discovery.req_initiated.get(),
         baseline_initiated + 3,
         "no new attempt before final deadline"
     );
     assert_eq!(
-        node.stats().discovery.resp_timed_out,
+        node.metrics().discovery.resp_timed_out.get(),
         baseline_timed_out,
         "no timeout before final deadline"
     );
@@ -1192,13 +1192,13 @@ async fn test_check_pending_lookups_default_sequence_unreachable() {
     );
     // resp_timed_out counter ticked.
     assert_eq!(
-        node.stats().discovery.resp_timed_out,
+        node.metrics().discovery.resp_timed_out.get(),
         baseline_timed_out + 1,
         "final timeout must increment discovery.resp_timed_out"
     );
     // No additional initiate_lookup on the timeout step.
     assert_eq!(
-        node.stats().discovery.req_initiated,
+        node.metrics().discovery.req_initiated.get(),
         baseline_initiated + 3,
         "the final-timeout step must NOT call initiate_lookup"
     );