Replace all UnsafeCell-based counters with safe alternatives:
- New ShardedCounter: per-thread TLS accumulation (thread_local crate)
with periodic publish to static AtomicU64. Zero atomic RMW on hot path.
- ACL RuleStatsTracker: remove unsafe Arc<RuleStats> raw pointer
mutation, use two ShardedCounter fields.
- ZCPacket: replace set_len on uninitialized BytesMut with
write_bytes + copy_nonoverlapping before set_len.
- StatsManager UnsafeCounter/MetricData: remove UnsafeCell and
unsafe impl Send/Sync, wrap ShardedCounter. last_updated uses
AtomicU64 epoch millis.
- Throughput: replace UnsafeCell with AtomicU64.
- secure_datagram: fix grace-window test timestamp.
