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/Conversion/VariantConverter.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Globalization;
+using System.Linq;
 using Google.Protobuf;
 using Google.Protobuf.WellKnownTypes;
 using MxGateway.Contracts.Proto;
@@ -118,6 +119,63 @@ public sealed class VariantConverter
         }
     }
 
+    /// <summary>
+    /// Converts an <see cref="MxValue"/> into a CLR object suitable for an
+    /// MXAccess COM write. The COM marshaler boxes the returned value into the
+    /// matching VARIANT, so this is the inverse of <see cref="Convert(object?)"/>.
+    /// </summary>
+    /// <param name="value">Protobuf value to convert.</param>
+    /// <returns>A COM-marshalable value, or <see langword="null"/> for an MXAccess null.</returns>
+    public object? ConvertToComValue(MxValue value)
+    {
+        if (value is null)
+        {
+            throw new ArgumentNullException(nameof(value));
+        }
+
+        if (value.IsNull)
+        {
+            return null;
+        }
+
+        return value.KindCase switch
+        {
+            MxValue.KindOneofCase.BoolValue => value.BoolValue,
+            MxValue.KindOneofCase.Int32Value => value.Int32Value,
+            MxValue.KindOneofCase.Int64Value => value.Int64Value,
+            MxValue.KindOneofCase.FloatValue => value.FloatValue,
+            MxValue.KindOneofCase.DoubleValue => value.DoubleValue,
+            MxValue.KindOneofCase.StringValue => value.StringValue,
+            // The COM marshaler renders a DateTime as VT_DATE; MXAccess accepts
+            // it as the timestamped-write time argument.
+            MxValue.KindOneofCase.TimestampValue => value.TimestampValue.ToDateTime(),
+            MxValue.KindOneofCase.ArrayValue => ConvertToComArray(value.ArrayValue),
+            MxValue.KindOneofCase.RawValue => throw new ArgumentException(
+                "MxValue raw payloads cannot be written to MXAccess.", nameof(value)),
+            _ => throw new ArgumentException(
+                "MxValue has no value kind set; nothing to write.", nameof(value)),
+        };
+    }
+
+    private static Array ConvertToComArray(MxArray array)
+    {
+        return array.ValuesCase switch
+        {
+            MxArray.ValuesOneofCase.BoolValues => array.BoolValues.Values.ToArray(),
+            MxArray.ValuesOneofCase.Int32Values => array.Int32Values.Values.ToArray(),
+            MxArray.ValuesOneofCase.Int64Values => array.Int64Values.Values.ToArray(),
+            MxArray.ValuesOneofCase.FloatValues => array.FloatValues.Values.ToArray(),
+            MxArray.ValuesOneofCase.DoubleValues => array.DoubleValues.Values.ToArray(),
+            MxArray.ValuesOneofCase.StringValues => array.StringValues.Values.ToArray(),
+            MxArray.ValuesOneofCase.TimestampValues =>
+                array.TimestampValues.Values.Select(timestamp => timestamp.ToDateTime()).ToArray(),
+            MxArray.ValuesOneofCase.RawValues => throw new ArgumentException(
+                "MxArray raw payloads cannot be written to MXAccess.", nameof(array)),
+            _ => throw new ArgumentException(
+                "MxArray has no element values set; nothing to write.", nameof(array)),
+        };
+    }
+
     private static MxValue ConvertScalar(
         object value,
         MxDataType expectedDataType)