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.toml

@@ -17,6 +17,7 @@ rand = "0.8"
 thiserror = "2.0"
 bech32 = "0.11"
 serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
 serde_yaml = "0.9"
 dirs = "6.0"
 hex = "0.4"
@@ -35,6 +36,10 @@ socket2 = { version = "0.5", features = ["all"] }
 tempfile = "3.15"
 criterion = { version = "0.5", features = ["html_reports"] }
 
+[[bin]]
+name = "fipsctl"
+path = "src/bin/fipsctl.rs"
+
 [[bench]]
 name = "bloom"
 harness = false