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.
fanyang