review(Driver.Modbus.Addressing): fix misleading byte-order hint + drop dead overflow guard

Re-review at 7286d320. -010 (Low): TryParseByteOrder no longer lists REAL/DINT/UINT as type
codes (gave wrong 'field 2' advice -> second parse error); generic byte-order error instead.
-011 (Low): remove unreachable offsetWithinBank>ushort.MaxValue guard (DecodeOctalVAddress
caps at 0xFFFF). + TDD.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Addressing/DirectLogicAddress.cs

@@ -117,11 +117,12 @@ public static class DirectLogicAddress
 
         // System bank: the registers are contiguous from V40400, so the offset within the bank
         // is the plain decimal distance from the octal base, not another octal decode.
-        var offsetWithinBank = octalValue - SystemVMemoryOctalBase;
-        if (offsetWithinBank > ushort.MaxValue)
-            throw new OverflowException(
-                $"V-memory address '{vAddress}' is outside the addressable system bank");
-        return SystemVMemoryToPdu((ushort)offsetWithinBank);
+        // Driver.Modbus.Addressing-011: the subtraction result is provably <= 0xBEFF because
+        // DecodeOctalVAddress already caps octalValue at 0xFFFF, so no overflow guard is needed
+        // here. The real overflow guard (pdu > ushort.MaxValue) lives in SystemVMemoryToPdu and
+        // is reachable only when that helper is called directly with a large explicit offset.
+        var offsetWithinBank = (ushort)(octalValue - SystemVMemoryOctalBase);
+        return SystemVMemoryToPdu(offsetWithinBank);
     }
 
     // Bit-memory bases per DL260 user manual §I/O-configuration.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Addressing/ModbusAddressParser.cs

@@ -503,13 +503,18 @@ public static class ModbusAddressParser
 
         if ((int)order == -1)
         {
-            // Driver.Modbus.Addressing-003: if the unknown token looks like a known type code
-            // (a 4-letter alphanumeric token that matches one of the recognised type strings),
-            // produce a diagnostic that directs the user to put the type in field 2, not field 3.
-            var isKnownTypeCode = text.ToUpperInvariant() is "BOOL" or "REAL" or "DINT" or "UINT"
-                || (text.Length <= 6 && text.StartsWith("STR", StringComparison.OrdinalIgnoreCase));
-            error = isKnownTypeCode
-                ? $"'{text}' looks like a type code; type belongs in field 2 (e.g. '40001:{text.ToUpperInvariant()}'), not field 3. Field 3 must be a 4-letter byte order (ABCD/CDAB/BADC/DCBA)"
+            // Driver.Modbus.Addressing-003: if the unknown token is a VALID type code in this
+            // parser, direct the user to put it in field 2 instead. Only BOOL (4-letter) and
+            // STR<n> are valid type codes that could appear here. "REAL", "DINT", "UINT" look
+            // like type names from other tools (STEP 7 / RSLogix) but are NOT valid in this
+            // parser's type table (Driver.Modbus.Addressing-010) — listing them here would give
+            // advice that leads to a second error ("Unknown type code 'REAL'").
+            var upper = text.ToUpperInvariant();
+            var isActualTypeCode = upper is "BOOL"
+                || (text.Length > 3 && text.StartsWith("STR", StringComparison.OrdinalIgnoreCase)
+                    && text[3..].All(char.IsDigit));
+            error = isActualTypeCode
+                ? $"'{text}' looks like a type code; type belongs in field 2 (e.g. '40001:{upper}'), not field 3. Field 3 must be a 4-letter byte order (ABCD/CDAB/BADC/DCBA)"
                 : $"Unknown byte order '{text}'. Valid: ABCD, CDAB, BADC, DCBA";
             return false;
         }