feat(modbus-rtu): add ModbusTransportMode enum (Tcp | RtuOverTcp)

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Contracts/ModbusTransportMode.cs

@@ -0,0 +1,17 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus;
+
+/// <summary>
+///     Wire transport for a Modbus driver instance. <see cref="Tcp"/> = Modbus/TCP (MBAP + TxId);
+///     <see cref="RtuOverTcp"/> = RTU framing (address + CRC-16, no MBAP) tunnelled over a socket to
+///     a serial→Ethernet gateway. Default <see cref="Tcp"/> — existing configs omit the field.
+///     A direct-serial <c>Rtu</c> member is intentionally NOT reserved here (descoped 2026-07-15);
+///     do not number-squat it.
+/// </summary>
+public enum ModbusTransportMode
+{
+    /// <summary>Modbus/TCP — 7-byte MBAP header + transaction id (the historical default).</summary>
+    Tcp,
+
+    /// <summary>RTU framing (<c>[addr][PDU][CRC-16]</c>) tunnelled over a socket to a serial→Ethernet gateway.</summary>
+    RtuOverTcp,
+}

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusTransportModeTests.cs

@@ -0,0 +1,15 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+public sealed class ModbusTransportModeTests
+{
+    [Fact]
+    public void Tcp_is_the_default_zero_member()
+        => ((ModbusTransportMode)0).ShouldBe(ModbusTransportMode.Tcp);
+
+    [Fact]
+    public void Exactly_two_members_ship_in_v1()
+        => Enum.GetNames<ModbusTransportMode>().ShouldBe(["Tcp", "RtuOverTcp"]);
+}