fix(twincat): dispose bit-RMW locks on shutdown + guard null parent read (review)

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs

@@ -169,6 +169,10 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
         _devices.Clear();
         _tagsByName.Clear();
         _resolver.Clear(); // drop transient equipment-tag parses so a config change re-parses
+        // Dispose + clear the per-parent-word RMW gates so ReinitializeAsync cycles don't orphan
+        // their SemaphoreSlim instances (each leaks a wait handle once contended).
+        foreach (var sem in _bitRmwLocks.Values) sem.Dispose();
+        _bitRmwLocks.Clear();
         _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
     }
 
@@ -319,9 +323,16 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
                         {
                             results[i] = new WriteResult(readStatus);
                         }
+                        else if (parentValue is null)
+                        {
+                            // Good status but no value — treating null as 0 would write a zeroed
+                            // word and clear every bit set on the device. Surface a Bad status and
+                            // write nothing rather than corrupt the parent word.
+                            results[i] = new WriteResult(TwinCATStatusMapper.BadCommunicationError);
+                        }
                         else
                         {
-                            var word = Convert.ToUInt32(parentValue ?? 0u);
+                            var word = Convert.ToUInt32(parentValue);
                             var updated = Convert.ToBoolean(w.Value) ? word | (1u << bit) : word & ~(1u << bit);
                             var writeStatus = await client.WriteValueAsync(
                                 parentPath, TwinCATDataType.UDInt, null, updated, cancellationToken).ConfigureAwait(false);
@@ -719,6 +730,10 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
         _devices.Clear();
         _tagsByName.Clear();
         _resolver.Clear(); // drop transient equipment-tag parses so a config change re-parses
+        // Dispose + clear the per-parent-word RMW gates (mirrors ShutdownAsync) so the
+        // SemaphoreSlim instances aren't orphaned on disposal.
+        foreach (var sem in _bitRmwLocks.Values) sem.Dispose();
+        _bitRmwLocks.Clear();
         _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
     }