Auto: ablegacy-3 — sub-element bit semantics

Closes #246

tests/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/FakeAbLegacyTag.cs

@@ -40,7 +40,25 @@ internal class FakeAbLegacyTag : IAbLegacyTagRuntime
     }
 
     public virtual int GetStatus() => Status;
-    public virtual object? DecodeValue(AbLegacyDataType type, int? bitIndex) => Value;
+    public int? LastDecodeBitIndex { get; private set; }
+    public AbLegacyDataType? LastDecodeType { get; private set; }
+    public virtual object? DecodeValue(AbLegacyDataType type, int? bitIndex)
+    {
+        LastDecodeType = type;
+        LastDecodeBitIndex = bitIndex;
+        // If the test seeded a parent-word value (ushort/short/int) and the driver asked for a
+        // specific status bit, mask it out so we can assert the correct bit reaches the client.
+        if (bitIndex is int bit && Value is not null and not bool)
+        {
+            try
+            {
+                var word = Convert.ToInt32(Value);
+                return ((word >> bit) & 1) != 0;
+            }
+            catch (Exception ex) when (ex is FormatException or InvalidCastException) { }
+        }
+        return Value;
+    }
     public virtual void EncodeValue(AbLegacyDataType type, int? bitIndex, object? value) => Value = value;
     public virtual void Dispose() => Disposed = true;
 }