merge: Modbus RTU-over-TCP transport (#495)

Wave 1 of the driver-expansion program. The branch was complete and live-gated but
had never been merged; it sat 15 commits ahead and 50 behind master.

Purely additive behind the existing IModbusTransport seam — the application protocol
(function codes, codecs, planner, coalescing, write path, probe, materialisation,
HistoryRead) is identical across TCP and RTU; only the wire framing differs
([addr][PDU][CRC-16], no MBAP, no TxId). Zero changes to codecs/planner/health.

Selected by a new Transport config field (ModbusTransportMode: Tcp | RtuOverTcp),
routed through ModbusTransportFactory (throws on unknown mode), with the string-enum
guard on options/DTO/factory and a Transport selector on the AdminUI Modbus form.
ModbusSocketLifecycle was extracted from ModbusTcpTransport (behaviour-preserving)
and is composed by both transports.

Descoped by design: direct-serial (System.IO.Ports) and Modbus ASCII — RTU buses are
reached exclusively via a serial->Ethernet gateway. No per-tag config changes; the
existing per-tag UnitId already makes a multi-drop bus work.

Re-validated against current master at merge time (the branch's own gate predates 50
commits of master): no overlapping files, clean merge, solution builds 0 errors, and
Modbus 344/344, Modbus.Addressing 161/161, AdminUI 740/740, Runtime 507/507,
Configuration 131/131 all pass. Confirmed the Transport selector is still reachable
through master's current authoring path (DriverConfigModal -> ModbusDriverForm), so
the branch's live /run gate result still applies.

tests/Server/ZB.MOM.WW.OtOpcUa.AdminUI.Tests/Uns/ModbusDriverFormModelTests.cs

@@ -61,4 +61,15 @@ public sealed class ModbusDriverFormModelTests
         json.ShouldContain("\"family\":\"MELSEC\"");           // enum-as-name (not a number), camelCase key
         json.ShouldNotContain("\"family\":0", Case.Sensitive); // never the numeric enum form
     }
+
+    [Fact]
+    public void Round_trip_preserves_Transport_mode()
+    {
+        var form = new ModbusDriverForm.FormModel { Transport = ModbusTransportMode.RtuOverTcp };
+        var json = JsonSerializer.Serialize(form.ToOptions(), JsonOpts);
+        var back = ModbusDriverForm.FormModel.FromOptions(
+            JsonSerializer.Deserialize<ModbusDriverOptions>(json, JsonOpts)!);
+        back.Transport.ShouldBe(ModbusTransportMode.RtuOverTcp);
+        json.ShouldContain("\"transport\":\"RtuOverTcp\""); // name string, never a number
+    }
 }