D2 follow-up: RTag2 doesn't cascade client identity to Trx

Tested hypothesis (1) from the plan: add RTag2(CM_EVENT tag id) to the
priming chain before AddNonStreamValuesBegin2.

Result:
- RTag2 itself succeeds: returns 25-byte response
  (01000000000100000001EE39C30EDCDC010100000000000000), no error buffer.
- But AddNonStreamValuesBegin2 still fails with the same
  04 33 00 00 00 (UnknownClient = 51) for all four handle formats.

So RTag2 on /Hist isn't the cross-service registration trigger we need
for /Trx. Plan doc updated with the result + next-session ordered
probes (try IStorageServiceContract, then IL walk CClientCommon,
then server-side decompile as last resort).

Probe orchestrator now also performs the RTag2 step so the test gives
one-shot diagnostic visibility of both calls.

178/178 tests pass.

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

src/AVEVA.Historian.Client/Wcf/HistorianWcfRevisionOrchestrator.cs

@@ -55,6 +55,29 @@ internal sealed class HistorianWcfRevisionOrchestrator
                 EndpointAddress retrievalEndpoint = HistorianWcfBindingFactory.CreateAuxiliaryEndpointAddress(_options, HistorianWcfServiceNames.Retrieval);
                 RunPrimingChain(historyChannel, context, auxBinding, statusEndpoint, transactionEndpoint, retrievalEndpoint);
 
+                // Hypothesis: calling RTag2 (RegisterTags2) cascades client identity into
+                // the Trx service's session table. The event flow uses RTag2 with the
+                // CM_EVENT tag id and subsequent ops succeed. Try RTag2 with that same
+                // tag id here as a registration probe.
+                try
+                {
+                    string handle = context.StorageSessionId.ToString("D").ToUpperInvariant();
+                    byte[] rtag2Buffer = BuildRTag2CmEventInputBuffer();
+                    bool rtag2Ok = historyChannel.RegisterTags2(
+                        handle: handle,
+                        elementCount: 1,
+                        inputBuffer: rtag2Buffer,
+                        outputBuffer: out byte[] rtag2Out,
+                        errorBuffer: out byte[] rtag2Err);
+                    result.RTag2Succeeded = rtag2Ok;
+                    result.RTag2OutHex = rtag2Out is null || rtag2Out.Length == 0 ? null : Convert.ToHexString(rtag2Out);
+                    result.RTag2ErrorHex = rtag2Err is null || rtag2Err.Length == 0 ? null : Convert.ToHexString(rtag2Err);
+                }
+                catch (Exception ex)
+                {
+                    result.RTag2Exception = $"{ex.GetType().Name}: {ex.Message}";
+                }
+
                 ChannelFactory<ITransactionServiceContract2> trxFactory = new(auxBinding, transactionEndpoint);
                 HistorianWcfClientCredentialsHelper.Configure(trxFactory, _options);
                 ITransactionServiceContract2 trxChannel = trxFactory.CreateChannel();
@@ -176,6 +199,22 @@ internal sealed class HistorianWcfRevisionOrchestrator
         }
     }
 
+    /// <summary>Same 24-byte RTag2 buffer the event flow uses (CM_EVENT tag id).</summary>
+    private static byte[] BuildRTag2CmEventInputBuffer()
+    {
+        byte[] buffer = new byte[24];
+        buffer[0] = 0x50;
+        buffer[1] = 0x67;
+        buffer[2] = 0x02;
+        buffer[3] = 0x00;
+        BinaryPrimitives.WriteUInt32LittleEndian(buffer.AsSpan(4, 4), 1u);
+        // CM_EVENT tag id — duplicated here to avoid a cross-class dependency on the
+        // event orchestrator. Verify against HistorianWcfEventOrchestrator.CmEventTagId
+        // if the value ever needs updating.
+        new Guid("353b8145-5df0-4d46-a253-871aef49b321").ToByteArray().CopyTo(buffer.AsSpan(8, 16));
+        return buffer;
+    }
+
     private static byte[] BuildUpdC3ClientStatusBlob()
     {
         byte[] blob = new byte[81];
@@ -219,6 +258,10 @@ internal sealed class HistorianRevisionProbeResult
     public string? BeginErrorHex { get; set; }
     public string? BeginException { get; set; }
     public List<HistorianRevisionBeginAttempt> BeginAttempts { get; } = new();
+    public bool RTag2Succeeded { get; set; }
+    public string? RTag2OutHex { get; set; }
+    public string? RTag2ErrorHex { get; set; }
+    public string? RTag2Exception { get; set; }
 }
 
 internal sealed class HistorianRevisionBeginAttempt