Phase 3 PR 53 -- Transport reconnect-on-drop + SO_KEEPALIVE for DL205 no-keepalive quirk. AutomationDirect H2-ECOM100 does NOT send TCP keepalives per docs/v2/dl205.md behavioral-oddities section -- any NAT/firewall device between the gateway and the PLC can silently close an idle socket after 2-5 minutes of inactivity. The PLC itself never notices and the first SendAsync after the drop would previously surface as IOException / EndOfStreamException / SocketException to the caller even though the PLC is perfectly healthy. PR 53 makes ModbusTcpTransport survive mid-session socket drops: SendAsync wraps the previous body as SendOnceAsync; on the first attempt, if the failure is a socket-layer error (IOException, SocketException, EndOfStreamException, ObjectDisposedException) AND autoReconnect is enabled (default true), the transport tears down the dead socket, calls ConnectAsync to re-establish, and resends the PDU exactly once. Deliberately single-retry -- further failures propagate so the driver health surface reflects the real state, no masking a dead PLC. Protocol-layer failures (e.g. ModbusException with exception code 02) are specifically NOT caught by the reconnect path -- they would just come back with the same exception code after the reconnect, so retrying is wasted wire time. Socket-level vs protocol-level is a discriminator inside IsSocketLevelFailure. Also enables SO_KEEPALIVE on the TcpClient with aggressive timing: TcpKeepAliveTime=30s, TcpKeepAliveInterval=10s, TcpKeepAliveRetryCount=3. Total time-to-detect-dead-socket = 30 + 10*3 = 60s, vs the Windows default 2-hour idle + 9 retries = 2h40min. Best-effort: older OSes that don't expose the fine-grained keepalive knobs silently skip them (catch {}). New ModbusDriverOptions.AutoReconnect bool (default true) threads through to the default transport factory in ModbusDriver -- callers wanting the old 'fail loud on drop' behavior can set AutoReconnect=false, or use a custom transportFactory that ignores the option. Unit tests: ModbusTcpReconnectTests boots a FlakeyModbusServer in-process (real TcpListener on loopback) that serves one valid FC03 response then forcibly shuts down the socket. Transport_recovers_from_mid_session_drop_and_retries_successfully issues two consecutive SendAsync calls and asserts both return valid PDUs -- the second must trigger the reconnect path transparently. Transport_without_AutoReconnect_propagates_drop_to_caller asserts the legacy behavior when the opt-out is taken. Validates real socket semantics rather than mocked exceptions. 142/142 Modbus.Tests pass (113 prior + 2 mapper + 2 reconnect + 25 accumulated across PRs 45-52); 11/11 DL205 integration tests still pass with MODBUS_SIM_PROFILE=dl205 -- no regression from the transport change.

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriverOptions.cs

@@ -45,6 +45,17 @@ public sealed class ModbusDriverOptions
     ///     by shortening the tag's <c>StringLength</c> or splitting it into multiple tags).
     /// </summary>
     public ushort MaxRegistersPerWrite { get; init; } = 123;
+
+    /// <summary>
+    ///     When <c>true</c> (default) the built-in <see cref="ModbusTcpTransport"/> detects
+    ///     mid-transaction socket failures (<see cref="System.IO.EndOfStreamException"/>,
+    ///     <see cref="System.Net.Sockets.SocketException"/>) and transparently reconnects +
+    ///     retries the PDU exactly once. Required for DL205/DL260 because the H2-ECOM100
+    ///     does not send TCP keepalives — intermediate NAT / firewall devices silently close
+    ///     idle sockets and the first send after the drop would otherwise surface as a
+    ///     connection error to the caller even though the PLC is up.
+    /// </summary>
+    public bool AutoReconnect { get; init; } = true;
 }
 
 public sealed class ModbusProbeOptions