Auto: ablegacy-7 — array contiguous block addressing

Closes #250

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

@@ -32,6 +32,11 @@ internal sealed class LibplctagLegacyTagRuntime : IAbLegacyTagRuntime
             Name = p.TagName,
             Timeout = p.Timeout,
         };
+        // PR 7 — array contiguous-block reads. Setting ElementCount tells libplctag to allocate
+        // a buffer covering N consecutive PCCC words (one frame, up to ~120 elements). The
+        // driver decodes element-by-element through DecodeArrayElement after a single ReadAsync.
+        if (p.ElementCount > 1)
+            _tag.ElementCount = p.ElementCount;
     }
 
     public Task InitializeAsync(CancellationToken cancellationToken) => _tag.InitializeAsync(cancellationToken);
@@ -127,6 +132,32 @@ internal sealed class LibplctagLegacyTagRuntime : IAbLegacyTagRuntime
         }
     }
 
+    /// <summary>
+    ///     PR 7 — decode element <paramref name="elementIndex"/> of an N-element contiguous
+    ///     PCCC block read. Element width is fixed per data type: Int / AnalogInt / Bit-as-word
+    ///     are 16-bit (2 bytes/element), Long / Float are 32-bit (4 bytes/element). Mirrors the
+    ///     non-array decoder shape but at byte offset <c>elementIndex × elementBytes</c>.
+    /// </summary>
+    public object? DecodeArrayElement(AbLegacyDataType type, int elementIndex)
+    {
+        if (elementIndex < 0) throw new ArgumentOutOfRangeException(nameof(elementIndex));
+        return type switch
+        {
+            // Bit / N-array reads — Rockwell convention is one BOOL per word (e.g. `B3:0,10`
+            // returns 10 BOOLs, not 160 individual bits). Each word is non-zero → true.
+            AbLegacyDataType.Bit => _tag.GetInt16(elementIndex * 2) != 0,
+            AbLegacyDataType.Int or AbLegacyDataType.AnalogInt => (int)_tag.GetInt16(elementIndex * 2),
+            AbLegacyDataType.Long => _tag.GetInt32(elementIndex * 4),
+            AbLegacyDataType.Float => _tag.GetFloat32(elementIndex * 4),
+            // String + element types are out-of-scope for PR 7 array reads — the PCCC layer's
+            // 240-byte frame ceiling means an ST array would only fit a couple of strings, and
+            // sub-element arrays (`T4:0,5.ACC`) are rejected at parse time. Surface a clear
+            // error if the driver mis-routes us here.
+            _ => throw new NotSupportedException(
+                $"AbLegacyDataType {type} cannot be decoded as a contiguous array element."),
+        };
+    }
+
     public void Dispose() => _tag.Dispose();
 
     private static PlcType MapPlcType(string attribute) => attribute switch