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/MxAccessComServer.cs

@@ -140,6 +140,74 @@ public sealed class MxAccessComServer : IMxAccessServer
         AsProxyServer4().AdviseSupervisory(serverHandle, itemHandle);
     }
 
+    /// <inheritdoc />
+    public void Write(
+        int serverHandle,
+        int itemHandle,
+        object? value,
+        int userId)
+    {
+        if (mxAccessComObject is IMxAccessServer typedFake)
+        {
+            typedFake.Write(serverHandle, itemHandle, value, userId);
+            return;
+        }
+
+        AsProxyServer().Write(serverHandle, itemHandle, value!, userId);
+    }
+
+    /// <inheritdoc />
+    public void Write2(
+        int serverHandle,
+        int itemHandle,
+        object? value,
+        object? timestamp,
+        int userId)
+    {
+        if (mxAccessComObject is IMxAccessServer typedFake)
+        {
+            typedFake.Write2(serverHandle, itemHandle, value, timestamp, userId);
+            return;
+        }
+
+        AsProxyServer4().Write2(serverHandle, itemHandle, value!, timestamp!, userId);
+    }
+
+    /// <inheritdoc />
+    public void WriteSecured(
+        int serverHandle,
+        int itemHandle,
+        int currentUserId,
+        int verifierUserId,
+        object? value)
+    {
+        if (mxAccessComObject is IMxAccessServer typedFake)
+        {
+            typedFake.WriteSecured(serverHandle, itemHandle, currentUserId, verifierUserId, value);
+            return;
+        }
+
+        AsProxyServer().WriteSecured(serverHandle, itemHandle, currentUserId, verifierUserId, value!);
+    }
+
+    /// <inheritdoc />
+    public void WriteSecured2(
+        int serverHandle,
+        int itemHandle,
+        int currentUserId,
+        int verifierUserId,
+        object? value,
+        object? timestamp)
+    {
+        if (mxAccessComObject is IMxAccessServer typedFake)
+        {
+            typedFake.WriteSecured2(serverHandle, itemHandle, currentUserId, verifierUserId, value, timestamp);
+            return;
+        }
+
+        AsProxyServer4().WriteSecured2(serverHandle, itemHandle, currentUserId, verifierUserId, value!, timestamp!);
+    }
+
     private ILMXProxyServer AsProxyServer()
     {
         return mxAccessComObject as ILMXProxyServer