Implement MXAccess write commands in the worker

The .proto contract and MxCommandKind already defined Write, Write2,
WriteSecured, and WriteSecured2, but the worker's MxAccessCommandExecutor
had no case for any of them — every write kind fell through to
CreateInvalidRequestReply ("Unsupported MXAccess command kind Write").

Implement all four:

- VariantConverter.ConvertToComValue projects an MxValue into a
  COM-marshalable object (scalars, arrays, null) — the inverse of the
  existing COM-to-MxValue projection.
- IMxAccessServer / MxAccessComServer gain Write/Write2/WriteSecured/
  WriteSecured2, routed to ILMXProxyServer / ILMXProxyServer4.
- MxAccessSession and MxAccessCommandExecutor add the four write paths,
  following the existing ExecuteAdvise pattern; the reply is a plain OK
  reply and the outcome surfaces later as an OnWriteComplete event.

Verified live: a Write now returns PROTOCOL_STATUS_CODE_OK and produces
an OnWriteComplete event where it previously returned InvalidRequest.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/MxGateway.Worker/MxAccess/MxAccessSession.cs

@@ -227,6 +227,78 @@ public sealed class MxAccessSession : IDisposable
             MxAccessAdviceKind.Supervisory);
     }
 
+    /// <summary>Writes a value to an item.</summary>
+    /// <param name="serverHandle">Handle returned by the worker.</param>
+    /// <param name="itemHandle">Handle returned by the worker.</param>
+    /// <param name="value">COM-marshalable value to write.</param>
+    /// <param name="userId">MXAccess user id (security classification) for the write.</param>
+    public void Write(
+        int serverHandle,
+        int itemHandle,
+        object? value,
+        int userId)
+    {
+        ThrowIfDisposed();
+
+        mxAccessServer.Write(serverHandle, itemHandle, value, userId);
+    }
+
+    /// <summary>Writes a value with an explicit source timestamp to an item.</summary>
+    /// <param name="serverHandle">Handle returned by the worker.</param>
+    /// <param name="itemHandle">Handle returned by the worker.</param>
+    /// <param name="value">COM-marshalable value to write.</param>
+    /// <param name="timestamp">COM-marshalable source timestamp for the write.</param>
+    /// <param name="userId">MXAccess user id (security classification) for the write.</param>
+    public void Write2(
+        int serverHandle,
+        int itemHandle,
+        object? value,
+        object? timestamp,
+        int userId)
+    {
+        ThrowIfDisposed();
+
+        mxAccessServer.Write2(serverHandle, itemHandle, value, timestamp, userId);
+    }
+
+    /// <summary>Performs a secured/verified write to an item.</summary>
+    /// <param name="serverHandle">Handle returned by the worker.</param>
+    /// <param name="itemHandle">Handle returned by the worker.</param>
+    /// <param name="currentUserId">MXAccess user id of the operator performing the write.</param>
+    /// <param name="verifierUserId">MXAccess user id of the verifier authorizing the write.</param>
+    /// <param name="value">COM-marshalable value to write.</param>
+    public void WriteSecured(
+        int serverHandle,
+        int itemHandle,
+        int currentUserId,
+        int verifierUserId,
+        object? value)
+    {
+        ThrowIfDisposed();
+
+        mxAccessServer.WriteSecured(serverHandle, itemHandle, currentUserId, verifierUserId, value);
+    }
+
+    /// <summary>Performs a secured/verified write with an explicit source timestamp.</summary>
+    /// <param name="serverHandle">Handle returned by the worker.</param>
+    /// <param name="itemHandle">Handle returned by the worker.</param>
+    /// <param name="currentUserId">MXAccess user id of the operator performing the write.</param>
+    /// <param name="verifierUserId">MXAccess user id of the verifier authorizing the write.</param>
+    /// <param name="value">COM-marshalable value to write.</param>
+    /// <param name="timestamp">COM-marshalable source timestamp for the write.</param>
+    public void WriteSecured2(
+        int serverHandle,
+        int itemHandle,
+        int currentUserId,
+        int verifierUserId,
+        object? value,
+        object? timestamp)
+    {
+        ThrowIfDisposed();
+
+        mxAccessServer.WriteSecured2(serverHandle, itemHandle, currentUserId, verifierUserId, value, timestamp);
+    }
+
     /// <summary>Adds multiple items in bulk, returning success/failure results.</summary>
     /// <param name="serverHandle">Handle returned by the worker.</param>
     /// <param name="tagAddresses">Enumerable of item definitions to add.</param>