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/mod.rs

@@ -8,6 +8,7 @@ mod acl;
 #[cfg(target_os = "linux")]
 mod ble;
 mod bloom;
+mod bloom_metrics;
 mod bloom_poison;
 mod bootstrap;
 mod decrypt_failure;
@@ -24,7 +25,14 @@ mod tcp;
 mod unit;
 
 pub(super) fn make_node() -> Node {
-    let config = Config::new();
+    make_node_with(Config::new())
+}
+
+/// Build a test node from an explicit `Config`. Prefer this over poking
+/// `node.config.*` after construction: immutable fields are mirrored into the
+/// shared `NodeContext` at build time, so a post-construction field poke is
+/// invisible to any reader that has migrated onto the `config()` accessor.
+pub(super) fn make_node_with(config: Config) -> Node {
     Node::new(config).unwrap()
 }