Task #136 — Modicon address-string parser (5/6-digit) + shared addressing assembly

Foundation for the Modbus addressing-grammar work tracked in #137-#145. Adds
ModbusModiconAddress.Parse / TryParse that turns classic Modicon strings
(40001 / 400001 / 30001 / 00001 / 10001) into (Region, ushort PduOffset).

Also extracts ModbusRegion to a new Driver.Modbus.Addressing assembly so the
Admin UI (#145) can reference the addressing surface without taking a dep on
the wire driver. The new assembly intentionally extends the same
ZB.MOM.WW.OtOpcUa.Driver.Modbus namespace as the driver — callers see the
type as if it lived in one place; only the project layout changes. No
existing call site needed editing (zero-churn move).

Behaviour:
- Single leading digit selects region (0=Coils, 1=DiscreteInputs,
  3=InputRegisters, 4=HoldingRegisters).
- 5-digit form: trailing 4 digits are 1-based register, supports 1..9999.
- 6-digit form: trailing 5 digits are 1-based register, supports 1..65536
  (full PDU address space).
- Strict 5-or-6 length check; whitespace trimmed; clear FormatException
  diagnostics for every malformed shape (wrong length, non-digit body,
  illegal leading digit, register zero, register overflow).

29/29 new unit tests pass. Full Driver.Modbus suite (182 tests) and the
solution-wide build still green after the ModbusRegion move.

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

@@ -112,8 +112,6 @@ public sealed record ModbusTagDefinition(
     ModbusStringByteOrder StringByteOrder = ModbusStringByteOrder.HighByteFirst,
     bool WriteIdempotent = false);
 
-public enum ModbusRegion { Coils, DiscreteInputs, InputRegisters, HoldingRegisters }
-
 public enum ModbusDataType
 {
     Bool,

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ZB.MOM.WW.OtOpcUa.Driver.Modbus.csproj

@@ -14,6 +14,7 @@
     <ItemGroup>
         <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Core.Abstractions\ZB.MOM.WW.OtOpcUa.Core.Abstractions.csproj"/>
         <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Core\ZB.MOM.WW.OtOpcUa.Core.csproj"/>
+        <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Driver.Modbus.Addressing\ZB.MOM.WW.OtOpcUa.Driver.Modbus.Addressing.csproj"/>
     </ItemGroup>
 
     <ItemGroup>