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/Supervision/PlcListenerSupervisor.cs

@@ -38,6 +38,7 @@ internal sealed partial class PlcListenerSupervisor : IAsyncDisposable
     private readonly ILogger<PlcListenerSupervisor> _logger;
     private readonly ResiliencePipeline? _backendConnectPipeline;
     private readonly Func<ReadCoalescingOptions>? _coalescingOptions;
+    private readonly Func<KeepaliveOptions>? _keepaliveOptions;
 
     // ── Mutable state ────────────────────────────────────────────────────────────────────
 
@@ -94,7 +95,8 @@ internal sealed partial class PlcListenerSupervisor : IAsyncDisposable
         ResiliencePipeline recoveryPipeline,
         ILogger<PlcListenerSupervisor> logger,
         ResiliencePipeline? backendConnectPipeline = null,
-        Func<ReadCoalescingOptions>? coalescingOptions = null)
+        Func<ReadCoalescingOptions>? coalescingOptions = null,
+        Func<KeepaliveOptions>? keepaliveOptions = null)
     {
         _plc                    = plc;
         _connectionOptions      = connectionOptions;
@@ -108,6 +110,7 @@ internal sealed partial class PlcListenerSupervisor : IAsyncDisposable
         _logger                 = logger;
         _backendConnectPipeline = backendConnectPipeline;
         _coalescingOptions      = coalescingOptions;
+        _keepaliveOptions       = keepaliveOptions;
     }
 
     /// <summary>
@@ -325,7 +328,8 @@ internal sealed partial class PlcListenerSupervisor : IAsyncDisposable
                     _pipeLogger,
                     _currentContext,
                     _backendConnectPipeline,
-                    _coalescingOptions);
+                    _coalescingOptions,
+                    _keepaliveOptions);
 
                 // Expose the current listener for status-page pair enumeration.
                 _currentListener = listener;