M3 R3.1: OpenStorageConnection is a dead end (error 85); precondition is front-door RegisterTags

Live-probed StorageService.OpenStorageConnection against the 2023 R2 server over a
write-enabled (0x401) session. Every attempt — sweeping ConnectionMode (0x401/0x402/0x1),
StorageSessionId-in (Open2-GUID / empty), and FreeDiskSpace — returns the IDENTICAL native
error type=4 code=85 ("session not registered"), so it's a structural refusal, not a bad
field value.

Decode (two corroborating facts):
- Error 85 is the same code the event read returns before RegisterTags2 (see
  HistorianWcfEventOrchestrator) — a generic "session not registered for this op".
- The 2023 R2 decompile shows OpenStorageConnection lives on a SEPARATE GrpcStorageClient
  (the storage engine's SF/snapshot channel, own port + service identity); HistorianAccess
  drives non-streamed writes through the native C++ HistorianClient, never this op.

So the roadmap's mapped "missing console session" step was wrong. The real non-streamed-write
precondition is the front-door HistoryService.RegisterTags (RTag2-family) for the target tag —
which is exactly why the R3.1 batch failed at AddNonStreamValues (no tag registered ->
StoreNonStreamValues had no route). Matches the original 2020-WCF D2 hypothesis.

Remaining (both need a native gRPC capture; do not guess bytes): the regular-tag RegisterTags
btTagInfos (only CM_EVENT's tag-GUID form is known) and the AddNonStreamValues btInput.

- HistorianGrpcStorageConnectionProbe + grpc-open-storage-connection CLI (opens nothing
  persistent; CloseStorageConnection on success)
- corrected revision-write-path.md §R3.1 follow-up + hcal-roadmap R3.1/R3.2 rows
- gated regression test pinning the error-85 refusal

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B6mcaT2PjRFKcogzp9UkfC

tests/AVEVA.Historian.Client.Tests/HistorianGrpcIntegrationTests.cs

@@ -148,6 +148,31 @@ public sealed class HistorianGrpcIntegrationTests
         Assert.True(result.EndDiscardSucceeded, "AddNonStreamValuesEnd(bCommit:false) should discard cleanly.");
     }
 
+    [Fact]
+    public async Task OpenStorageConnection_OverGrpc_RefusedAsNotRegistered()
+    {
+        string? host = Environment.GetEnvironmentVariable("HISTORIAN_GRPC_HOST");
+        if (string.IsNullOrWhiteSpace(host) || string.IsNullOrEmpty(Environment.GetEnvironmentVariable("HISTORIAN_USER")))
+        {
+            return;
+        }
+
+        // M3 R3.1 follow-up finding (2026-06-21): StorageService.OpenStorageConnection is NOT the
+        // missing non-streamed-write precondition. It's the storage engine's SF/snapshot channel
+        // (separate GrpcStorageClient / service identity), and on the Historian front door it is
+        // refused with native type=4 code=85 ("session not registered") for every parameter combo —
+        // the same code the event read returns before RegisterTags2. The real precondition is the
+        // front-door HistoryService.RegisterTags (RTag2-family). See docs/plans/revision-write-path.md
+        // §"R3.1 follow-up". This test pins the refusal so a future server/behaviour change is noticed.
+        var probe = new HistorianGrpcStorageConnectionProbe(BuildOptions(host));
+        HistorianGrpcOpenStorageConnectionResult result = await probe.ProbeAsync(CancellationToken.None);
+
+        Assert.True(result.OpenSucceeded, "the write-enabled gRPC session itself should still open.");
+        Assert.False(result.OpenStorageSucceeded, "OpenStorageConnection is not a front-door client op (error 85).");
+        Assert.NotEmpty(result.Attempts);
+        Assert.All(result.Attempts, a => Assert.False(a.Succeeded));
+    }
+
     private static HistorianClientOptions BuildOptions(string host)
     {
         string? user = Environment.GetEnvironmentVariable("HISTORIAN_USER");