Auto: ablegacy-5 — PD/MG/PLS/BT structure files

Adds PD (PID), MG (Message), PLS (Programmable Limit Switch) and BT
(Block Transfer) file types to the PCCC parser. New AbLegacyDataType
enum members (PidElement / MessageElement / PlsElement /
BlockTransferElement) plus per-type sub-element catalogue:

  - PD: SP/PV/CV/KP/KI/KD/MAXS/MINS/DB/OUT as Float32; EN/DN/MO/PE/
        AUTO/MAN/SP_VAL/SP_LL/SP_HL as Boolean (word-0 status bits).
  - MG: RBE/MS/SIZE/LEN as Int32; EN/EW/ER/DN/ST/CO/NR/TO as Boolean.
  - PLS: LEN as Int32 (bit table varies per PLC).
  - BT: RLEN/DLEN as Int32; EN/ST/DN/ER/CO/EW/TO/NR as Boolean.

Per-family flags on AbLegacyPlcFamilyProfile gate availability:

  - PD/MG: SLC500 + PLC-5 (operator + status bits both present).
  - PLS/BT: PLC-5 only (chassis-IO block transfer is PLC-5-specific).
  - MicroLogix + LogixPccc: rejected — no legacy file-letter form.

Status-bit indices match Rockwell DTAM / 1747-RM001 / 1785-6.5.12:
PD word 0 bits 0-8, MG/BT word 0 bits 8-15. PLC-set status bits
(PE/DN/SP_*; ST/DN/ER/CO/EW/NR/TO) are surfaced as ViewOnly via
IsPlcSetStatusBit, matching the Timer/Counter/Control pattern from
ablegacy-3.

LibplctagLegacyTagRuntime decodes PD non-bit members as Float32 and
MG/BT/PLS non-bit members as Int32; status bits route through GetBit
with the bit-position encoded by the driver via StatusBitIndex.

Tests: parser positive cases per family + negative cases per family,
catalogue + bit-index + read-only-bit assertions.

Closes #248

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

@@ -156,9 +156,19 @@ public sealed record AbLegacyAddress(
 
         // Reject unknown file letters — these cover SLC/ML/PLC-5 canonical families.
         // Function-file letters (RTC/HSC/DLS/MMI/PTO/PWM/STI/EII/IOS/BHI) are MicroLogix-only.
+        // Structure-file letters (PD/MG/PLS/BT) are gated per family — PD/MG are common on
+        // SLC500 + PLC-5; PLS/BT are PLC-5 only. MicroLogix and LogixPccc reject them.
         if (!IsKnownFileLetter(letter))
         {
-            if (!IsFunctionFileLetter(letter) || profile?.SupportsFunctionFiles != true) return null;
+            if (IsFunctionFileLetter(letter))
+            {
+                if (profile?.SupportsFunctionFiles != true) return null;
+            }
+            else if (IsStructureFileLetter(letter))
+            {
+                if (!StructureFileSupported(letter, profile)) return null;
+            }
+            else return null;
         }
 
         var octalForIo = profile?.OctalIoAddressing == true && (letter == "I" || letter == "O");
@@ -273,4 +283,28 @@ public sealed record AbLegacyAddress(
         "RTC" or "HSC" or "DLS" or "MMI" or "PTO" or "PWM" or "STI" or "EII" or "IOS" or "BHI" => true,
         _ => false,
     };
+
+    /// <summary>
+    ///     Structure-file prefixes added in #248: PD (PID), MG (Message), PLS (Programmable Limit
+    ///     Switch), BT (Block Transfer). Per-family availability is gated by the matching
+    ///     <c>Supports*File</c> flag on <see cref="AbLegacyPlcFamilyProfile"/>.
+    /// </summary>
+    internal static bool IsStructureFileLetter(string letter) => letter switch
+    {
+        "PD" or "MG" or "PLS" or "BT" => true,
+        _ => false,
+    };
+
+    private static bool StructureFileSupported(string letter, AbLegacyPlcFamilyProfile? profile)
+    {
+        if (profile is null) return false;
+        return letter switch
+        {
+            "PD" => profile.SupportsPidFile,
+            "MG" => profile.SupportsMessageFile,
+            "PLS" => profile.SupportsPlsFile,
+            "BT" => profile.SupportsBlockTransferFile,
+            _ => false,
+        };
+    }
 }