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/install/mbproxy.config.template.json

@@ -99,7 +99,34 @@
       // Max time (ms) to wait for in-flight PDUs to complete during graceful shutdown
       // (sc.exe stop / Windows Service stop signal). After this deadline the coordinator
       // cancels remaining work and proceeds. Keep at or below the SCM wait-hint (30 s).
-      "GracefulShutdownTimeoutMs": 10000
+      "GracefulShutdownTimeoutMs": 10000,
+
+      // ── Keepalive / connection monitoring ───────────────────────────────────
+      // The DL205/DL260 ECOM does not emit TCP keepalives, so an idle backend
+      // socket can be silently dropped by a middlebox (switch, firewall, NAT)
+      // after 2-5 minutes. This section enables OS-level SO_KEEPALIVE on both
+      // backend and upstream sockets, and drives a periodic Modbus FC03 heartbeat
+      // on each idle backend socket so a dead path is detected before a real
+      // client request hits it. See docs/Architecture/Keepalive.md.
+      "Keepalive": {
+        // Master switch. false → no SO_KEEPALIVE and no heartbeat; the proxy
+        // behaves exactly as a pre-keepalive build.
+        "Enabled": true,
+
+        // SO_KEEPALIVE: idle time (ms) before the OS sends its first probe.
+        "TcpIdleTimeMs": 30000,
+        // SO_KEEPALIVE: interval (ms) between probes once the idle time elapses.
+        "TcpProbeIntervalMs": 5000,
+        // SO_KEEPALIVE: unanswered probes before the OS declares the socket dead.
+        "TcpProbeCount": 4,
+
+        // Backend heartbeat: after this much backend idle (ms) the proxy issues a
+        // synthetic FC03 qty=1 read to keep the path warm and prove the ECOM is
+        // still answering Modbus. Must be greater than BackendRequestTimeoutMs.
+        "BackendHeartbeatIdleMs": 30000,
+        // FC03 PDU address the heartbeat reads. 0 = V0, valid on DL205/DL260.
+        "BackendHeartbeatProbeAddress": 0
+      }
     },
 
     // ── Resilience policies ─────────────────────────────────────────────────────────────