mbproxy: add keepalive / connection monitoring

The DL205/DL260 ECOM emits no TCP keepalives, so an idle backend socket
can be silently dropped by a middlebox (switch, firewall, NAT) after
2-5 minutes. Enable OS SO_KEEPALIVE on backend and accepted upstream
sockets, and drive a periodic synthetic FC03 heartbeat on each idle
backend socket so a dead path is detected before a real client request
hits it. Controlled by Connection.Keepalive (ON by default).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/src/Mbproxy/Proxy/Multiplexing/UpstreamPipe.cs

@@ -1,6 +1,7 @@
 using System.Net;
 using System.Net.Sockets;
 using System.Threading.Channels;
+using Mbproxy.Options;
 
 namespace Mbproxy.Proxy.Multiplexing;
 
@@ -77,10 +78,15 @@ internal sealed partial class UpstreamPipe : IAsyncDisposable
     /// </summary>
     public bool IsAlive => !_disposed && !_cts.IsCancellationRequested;
 
-    public UpstreamPipe(Socket upstream, string plcName, ILogger logger)
+    public UpstreamPipe(Socket upstream, string plcName, ILogger logger, KeepaliveOptions? keepalive = null)
     {
         _upstream = upstream;
         _upstream.NoDelay = true;
+        // Enable OS TCP keepalive on the accepted client socket so a half-open/dead
+        // client (gone without a TCP FIN) faults the read loop and is reaped, instead of
+        // leaking a pipe + correlation slots until the proxy next tries to write to it.
+        if (keepalive is not null)
+            SocketKeepalive.Apply(_upstream, keepalive);
         RemoteEp = upstream.RemoteEndPoint as IPEndPoint;
         _plcName = plcName;
         _logger  = logger;