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.Tests/MxAccess/MxAccessComServerTests.cs

@@ -134,5 +134,26 @@ public sealed class MxAccessComServerTests
         {
             calls.Add($"AdviseSupervisory:{serverHandle}:{itemHandle}");
         }
+
+        public void Write(int serverHandle, int itemHandle, object? value, int userId)
+        {
+            calls.Add($"Write:{serverHandle}:{itemHandle}:{value}:{userId}");
+        }
+
+        public void Write2(int serverHandle, int itemHandle, object? value, object? timestamp, int userId)
+        {
+            calls.Add($"Write2:{serverHandle}:{itemHandle}:{value}:{timestamp}:{userId}");
+        }
+
+        public void WriteSecured(int serverHandle, int itemHandle, int currentUserId, int verifierUserId, object? value)
+        {
+            calls.Add($"WriteSecured:{serverHandle}:{itemHandle}:{currentUserId}:{verifierUserId}:{value}");
+        }
+
+        public void WriteSecured2(
+            int serverHandle, int itemHandle, int currentUserId, int verifierUserId, object? value, object? timestamp)
+        {
+            calls.Add($"WriteSecured2:{serverHandle}:{itemHandle}:{currentUserId}:{verifierUserId}:{value}:{timestamp}");
+        }
     }
 }