test(bench): add counter contention benchmark

Adds benches/counter_contention.rs comparing the production CounterHandle
against reconstructed baselines (pre-optimization single-atomic + CAS +
Mutex<Instant>) under tokio-task contention, plus counter-only variants.
Uses the real stats_manager types so the numbers reflect shipped code.

Placed between the sharding and fastant commits so each can be benchmarked
independently: at this commit prod = sharded counter + Mutex<Instant> touch.

easytier/Cargo.toml

@@ -342,6 +342,11 @@ futures-util = "0.3.31"
 maplit = "1.0.2"
 tempfile = "3.22.0"
 ctor = "0.8.0"
+criterion = { version = "0.5", features = ["html_reports"] }
+
+[[bench]]
+name = "counter_contention"
+harness = false
 
 [target.'cfg(target_os = "linux")'.dev-dependencies]
 defguard_wireguard_rs = "0.4.2"