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/ProxyWorker.cs

@@ -150,6 +150,11 @@ internal sealed partial class ProxyWorker : BackgroundService
             Func<ReadCoalescingOptions> coalescingAccessor =
                 () => _options.CurrentValue.Resilience.ReadCoalescing;
 
+            // Live accessor for KeepaliveOptions so a hot-reload of `Connection.Keepalive`
+            // propagates to the backend heartbeat loop and to upstream-socket keepalive.
+            Func<KeepaliveOptions> keepaliveAccessor =
+                () => _options.CurrentValue.Connection.Keepalive;
+
             var supervisor = new PlcListenerSupervisor(
                 plc,
                 opts.Connection,
@@ -161,7 +166,8 @@ internal sealed partial class ProxyWorker : BackgroundService
                 recoveryPipeline,
                 _loggerFactory.CreateLogger<PlcListenerSupervisor>(),
                 backendPipeline,
-                coalescingAccessor);
+                coalescingAccessor,
+                keepaliveAccessor);
 
             _supervisors[plc.Name] = supervisor;
         }
@@ -175,7 +181,9 @@ internal sealed partial class ProxyWorker : BackgroundService
         // (add/restart paths) honour hot-reloaded ReadCoalescing values.
         Func<ReadCoalescingOptions> reconcilerCoalescingAccessor =
             () => _options.CurrentValue.Resilience.ReadCoalescing;
-        _reconciler.Attach(_supervisors, opts, reconcilerCoalescingAccessor);
+        Func<KeepaliveOptions> reconcilerKeepaliveAccessor =
+            () => _options.CurrentValue.Connection.Keepalive;
+        _reconciler.Attach(_supervisors, opts, reconcilerCoalescingAccessor, reconcilerKeepaliveAccessor);
 
         if (_supervisors.Count == 0)
         {