fix(abcip): stride user-string arrays by the read buffer, not the built-in STRING size

Live read-only check against a production CompactLogix press: Program:UN01_Press._ModeNames
(STR0016[32], 4 elements requested) returned an 80-byte buffer with libplctag.NET 1.5.2's
ElementSize unset. The driver fell back to the static 88-byte STRING stride, read element
1 past the buffer, and the whole read failed (ErrorOutOfBounds -> BadCommunicationError).
When the buffer divides evenly by the element count, that quotient is now the stride;
the read returns [undefined, Production, Maintenance, Manual] as the controller holds.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/LibplctagTagRuntime.cs

@@ -79,8 +79,7 @@ internal sealed class LibplctagTagRuntime : IAbCipTagRuntime
         // accounts for STRING capacity + atomic widths. ElementSize is int? on the wrapper — fall
         // back to a static size table when it is null/<=0 (e.g. a zero-length read) so the stride
         // is never 0.
-        var elementSize = _tag.ElementSize ?? 0;
-        var stride = elementSize > 0 ? elementSize : ElementByteSize(type);
+        var stride = ResolveArrayStride(_tag.ElementSize, _tag.GetSize(), count, type);
 
         return type switch
         {
@@ -99,6 +98,26 @@ internal sealed class LibplctagTagRuntime : IAbCipTagRuntime
         };
     }
 
+    /// <summary>
+    ///     Byte stride between array elements. libplctag's <c>ElementSize</c> wins when populated, but
+    ///     after a read it is <c>null</c> (observed live, libplctag.NET 1.5.2) — and the static table
+    ///     assumes the built-in 88-byte STRING, which is wrong for a user string type: a <c>STR0016</c>
+    ///     element is 20 bytes, so element 1 was read at offset 88 of an 80-byte buffer and the whole
+    ///     read failed. When the read buffer divides evenly by the element count, that quotient IS the
+    ///     element size, so it is used before the static table.
+    /// </summary>
+    /// <param name="elementSize">libplctag's reported element size, if any.</param>
+    /// <param name="bufferSize">Bytes in the read buffer.</param>
+    /// <param name="count">Element count requested.</param>
+    /// <param name="type">Element type.</param>
+    /// <returns>The stride in bytes.</returns>
+    internal static int ResolveArrayStride(int? elementSize, int bufferSize, int count, AbCipDataType type)
+    {
+        if (elementSize is > 0) return elementSize.Value;
+        if (count > 0 && bufferSize > 0 && bufferSize % count == 0) return bufferSize / count;
+        return ElementByteSize(type);
+    }
+
     private bool[] BuildBoolArray(int count)
     {
         var arr = new bool[count];