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/spanning_tree.rs

@@ -832,7 +832,7 @@ async fn test_rejects_tree_announce_with_inconsistent_root() {
         .coords()
         .unwrap()
         .clone();
-    let accepted_before = nodes[1].node.stats().tree.accepted;
+    let accepted_before = nodes[1].node.metrics().tree.accepted.get();
 
     // Use two fixed synthetic ancestors so the forged path is explicit:
     // - fake_parent = 00000000000000000000000000000000
@@ -877,7 +877,7 @@ async fn test_rejects_tree_announce_with_inconsistent_root() {
         nodes[1].node.tree_state().my_coords().depth(),
         current_depth
     );
-    assert_eq!(nodes[1].node.stats().tree.accepted, accepted_before);
+    assert_eq!(nodes[1].node.metrics().tree.accepted.get(), accepted_before);
     assert_eq!(
         nodes[1].node.get_peer(&a_addr).unwrap().coords().unwrap(),
         &peer_coords_before