Auto: ablegacy-2 — MicroLogix function-file letters

Closes #245

src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyAddress.cs

@@ -107,7 +107,11 @@ public sealed record AbLegacyAddress(
         }
 
         // Reject unknown file letters — these cover SLC/ML/PLC-5 canonical families.
-        if (!IsKnownFileLetter(letter)) return null;
+        // Function-file letters (RTC/HSC/DLS/MMI/PTO/PWM/STI/EII/IOS/BHI) are MicroLogix-only.
+        if (!IsKnownFileLetter(letter))
+        {
+            if (!IsFunctionFileLetter(letter) || profile?.SupportsFunctionFiles != true) return null;
+        }
 
         var octalForIo = profile?.OctalIoAddressing == true && (letter == "I" || letter == "O");
 
@@ -151,4 +155,14 @@ public sealed record AbLegacyAddress(
         "N" or "F" or "B" or "L" or "ST" or "T" or "C" or "R" or "I" or "O" or "S" or "A" => true,
         _ => false,
     };
+
+    /// <summary>
+    ///     MicroLogix 1100/1400 function-file prefixes. Each maps to a single fixed instance with a
+    ///     known sub-element catalogue (see <see cref="AbLegacyDataType"/>).
+    /// </summary>
+    internal static bool IsFunctionFileLetter(string letter) => letter switch
+    {
+        "RTC" or "HSC" or "DLS" or "MMI" or "PTO" or "PWM" or "STI" or "EII" or "IOS" or "BHI" => true,
+        _ => false,
+    };
 }