Add Unix domain control socket for runtime observability

Add a Unix domain socket interface for querying node state at runtime.
A spawned tokio task accepts connections and communicates with the main
event loop via mpsc/oneshot channels, keeping all Node access
single-threaded.

Includes:
- src/control/ module with socket lifecycle, JSON protocol, and 11
  query handlers (status, peers, links, tree, sessions, bloom, mmp,
  cache, connections, transports, routing)
- Separate fipsctl binary for CLI queries (fipsctl show <command>)
- ControlConfig in node configuration (enabled, socket_path)
- Integration into the main select! event loop

Cargo.lock

@@ -543,6 +543,7 @@ dependencies = [
  "rtnetlink",
  "secp256k1",
  "serde",
+ "serde_json",
  "serde_yaml",
  "sha2",
  "simple-dns",