feat(batch45): implement events server methods — stats, remote tracking, connection events

Port 80 features from server/events.go including the full events infrastructure:
internal send/receive loops, system subscription machinery, statsz heartbeats,
remote server tracking, connection event advisories, user-info handler, OCSP peer
reject events, remote latency merge, kick/ldm client, and helper functions.

Add ClearConnectionHeartbeatTimer/SetConnectionHeartbeatTimer to Account,
add MsgHandler/SysMsgHandler delegates and supporting types (ServerApiResponse,
EventFilterOptions, StatszEventOptions, UserInfo, KickClientReq, LdmClientReq,
AccNumSubsReq) to EventTypes.cs, and add Seq field to ServerInfo for heartbeat
sequence tracking.

dotnet/src/ZB.MOM.NatsNet.Server/Accounts/Account.cs

@@ -4507,6 +4507,28 @@ public sealed partial class Account : INatsAccount
         return string.Empty;
     }
 
+    /// <summary>
+    /// Clears the connection-heartbeat timer. Caller must hold the account lock.
+    /// Mirrors Go <c>(a *Account) clearConnectionTimer()</c> in server/events.go.
+    /// </summary>
+    internal void ClearConnectionHeartbeatTimer()
+    {
+        ClearTimerLocked(ref _ctmr);
+    }
+
+    /// <summary>
+    /// Starts or resets the connection-heartbeat timer.
+    /// Caller must hold the account lock.
+    /// Mirrors Go inline timer setup in <c>sendAccConnsUpdate()</c>.
+    /// </summary>
+    internal void SetConnectionHeartbeatTimer(long delayMs, Action callback)
+    {
+        if (_ctmr == null)
+            _ctmr = new Timer(_ => callback(), null, delayMs, Timeout.Infinite);
+        else
+            _ctmr.Change(delayMs, Timeout.Infinite);
+    }
+
     /// <summary>
     /// Stops and nulls out a timer. Lock must be held by the caller.
     /// Mirrors Go <c>clearTimer(t **time.Timer)</c>.