feat: add ClientFlags bitfield with thread-safe holder

src/NATS.Server/ClientFlags.cs

@@ -0,0 +1,41 @@
+namespace NATS.Server;
+
+/// <summary>
+/// Connection state flags tracked per client.
+/// Corresponds to Go server/client.go clientFlag bitfield.
+/// Thread-safe via Interlocked operations on the backing int.
+/// </summary>
+[Flags]
+public enum ClientFlags
+{
+    ConnectReceived = 1 << 0,
+    FirstPongSent = 1 << 1,
+    HandshakeComplete = 1 << 2,
+    CloseConnection = 1 << 3,
+    WriteLoopStarted = 1 << 4,
+    IsSlowConsumer = 1 << 5,
+    ConnectProcessFinished = 1 << 6,
+}
+
+/// <summary>
+/// Thread-safe holder for client flags using Interlocked operations.
+/// </summary>
+public sealed class ClientFlagHolder
+{
+    private int _flags;
+
+    public void SetFlag(ClientFlags flag)
+    {
+        Interlocked.Or(ref _flags, (int)flag);
+    }
+
+    public void ClearFlag(ClientFlags flag)
+    {
+        Interlocked.And(ref _flags, ~(int)flag);
+    }
+
+    public bool HasFlag(ClientFlags flag)
+    {
+        return (Volatile.Read(ref _flags) & (int)flag) != 0;
+    }
+}