review(Driver.Historian.Wonderware.Client): async frame-header write + wire-parity test

Re-review at 7286d320. -011: FrameWriter folded the sync WriteByte (could block on SslStream
past the call timeout) into one async 5-byte header write. -012: DefaultTcpConnectFactory
readonly. -013: wire-parity test for PerEventStatus [Key(4)]. No wire change.

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Wonderware.Client.Tests/ContractsWireParityTests.cs

@@ -183,7 +183,7 @@ public sealed class ContractsWireParityTests
         rt.Events[0].EventTimeUtcTicks.ShouldBe(999L);
     }
 
-    /// <summary>Verifies that WriteAlarmEventsReply round-trips correctly.</summary>
+    /// <summary>Verifies that WriteAlarmEventsReply round-trips correctly (legacy PerEventOk path).</summary>
     [Fact]
     public void WriteAlarmEventsReply_RoundTrips()
     {
@@ -202,6 +202,36 @@ public sealed class ContractsWireParityTests
         rt.PerEventOk.ShouldBe(new[] { true, false, true });
     }
 
+    /// <summary>
+    ///     Pins the <c>[Key(4)]</c> index for <see cref="WriteAlarmEventsReply.PerEventStatus"/>,
+    ///     the additive granular status field added in the <c>feddc2b8</c> commit. A silent
+    ///     Key-index drift in either the client or the sidecar mirror copy would swap the legacy
+    ///     <c>PerEventOk</c> bool array and the new status byte array, misclassifying outcomes
+    ///     at runtime. (Finding 013.)
+    /// </summary>
+    [Fact]
+    public void WriteAlarmEventsReply_PerEventStatus_IsAtKey4_AndRoundTrips()
+    {
+        var original = new WriteAlarmEventsReply
+        {
+            CorrelationId = "s",
+            Success = true,
+            PerEventOk = [true],
+            PerEventStatus = [0, 1, 2],  // Ack, Retry, Permanent
+        };
+        var bytes = MessagePackSerializer.Serialize(original);
+
+        // The array must start with fixarray(5) — five keys at indices 0-4.
+        bytes[0].ShouldBe((byte)0x95, "WriteAlarmEventsReply must be a 5-field MessagePack array");
+
+        var rt = MessagePackSerializer.Deserialize<WriteAlarmEventsReply>(bytes);
+        rt.CorrelationId.ShouldBe("s");
+        rt.Success.ShouldBeTrue();
+        rt.PerEventOk.ShouldBe(new[] { true });
+        // Key(4): PerEventStatus must round-trip independently of Key(3): PerEventOk.
+        rt.PerEventStatus.ShouldBe(new byte[] { 0, 1, 2 });
+    }
+
     // ---- MessageKind enum values are pinned ----
     // Changing a MessageKind value is a wire break; pin them explicitly.