R4.3: measured idle-state GetStoreForwardStatusAsync over gRPC

Route GetStoreForwardStatusAsync to a gRPC path that actually contacts the
server (StatusService.GetHistorianConsoleStatus) instead of synthesizing an
all-false result blind. On a reachable/normal server it returns the
not-storing baseline but MEASURED; when the server is unreachable or the
console-status call fails it reports ErrorOccurred with the underlying error
(the old synthesis never contacted the server). The active-SF buffer
magnitude (Storing/Pending/DataStored) stays false because it lives behind
the D2 storage-engine console wall.

Non-gRPC transports keep the synthesized fallback. Live-verified against the
2023 R2 server; gated integration test
GetStoreForwardStatusAsync_OverGrpc_ReturnsMeasuredIdleState added. README
operation table updated.

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

src/AVEVA.Historian.Client/Grpc/HistorianGrpcStatusClient.cs

@@ -1,4 +1,5 @@
 using Grpc.Core;
+using AVEVA.Historian.Client.Models;
 using GrpcStatus = ArchestrA.Grpc.Contract.Status;
 
 namespace AVEVA.Historian.Client.Grpc;
@@ -36,6 +37,78 @@ internal static class HistorianGrpcStatusClient
         return (response.Status?.BSuccess ?? false) ? response.StrParameterValue : null;
     }
 
+    /// <summary>
+    /// Returns a <em>measured</em> store-forward status over the 2023 R2 gRPC transport (R4.3).
+    /// <para>
+    /// Unlike the non-gRPC <see cref="Wcf.HistorianWcfStatusClient"/> path — which synthesizes an
+    /// all-false result <em>without contacting the server</em> — this opens an authenticated session
+    /// and calls <c>StatusService.GetHistorianConsoleStatus</c>, the only SF-adjacent signal reachable
+    /// from a pure managed client. (The direct <c>StorageService.GetSFParameter</c> /
+    /// <c>GetRemainingSnapshotsSize</c> RPCs that carry the SF buffer magnitude require the
+    /// <c>OpenStorageConnection</c> storage-engine console handle, which is gated behind the D2
+    /// storage-engine-pipe wall and is unobtainable here — see
+    /// <c>docs/plans/store-forward-cache-reverse-engineering.md</c> §9.7.)
+    /// </para>
+    /// <para>
+    /// Semantics: a successful console-status read means the server is reachable and its storage
+    /// console is reporting normally ⇒ the not-storing baseline (all flags false), but now
+    /// <em>measured</em> rather than blindly assumed. If the server cannot be reached/authenticated,
+    /// or the console-status call itself fails, <see cref="HistorianStoreForwardStatus.ErrorOccurred"/>
+    /// is set with the underlying error. The active-SF state (<see cref="HistorianStoreForwardStatus.Storing"/>
+    /// / <see cref="HistorianStoreForwardStatus.Pending"/> / <see cref="HistorianStoreForwardStatus.DataStored"/>
+    /// magnitude) is NOT observable from this signal and remains false; populating it requires the
+    /// D2-gated storage-console path.
+    /// </para>
+    /// </summary>
+    public static Task<HistorianStoreForwardStatus> GetStoreForwardStatusAsync(
+        HistorianClientOptions options,
+        CancellationToken cancellationToken)
+        => Task.Run(() => GetStoreForwardStatus(options, cancellationToken), cancellationToken);
+
+    private static HistorianStoreForwardStatus GetStoreForwardStatus(HistorianClientOptions options, CancellationToken cancellationToken)
+    {
+        HistorianStoreForwardStatus NotStoring(bool errorOccurred, string? error) => new(
+            ServerName: options.Host,
+            Pending: false,
+            ErrorOccurred: errorOccurred,
+            Error: error,
+            DataStored: false,
+            Storing: false,
+            ConnectionKind: HistorianConnectionKind.Process);
+
+        try
+        {
+            using HistorianGrpcConnection connection = HistorianGrpcChannelFactory.Create(options);
+            HistorianGrpcHandshake.Session session = HistorianGrpcHandshake.OpenSession(connection, options, cancellationToken);
+
+            var statusClient = new GrpcStatus.StatusService.StatusServiceClient(connection.Channel);
+            GrpcStatus.GetHistorianConsoleStatusResponse response = statusClient.GetHistorianConsoleStatus(
+                new GrpcStatus.GetHistorianConsoleStatusRequest { StrHandle = session.StringHandle },
+                connection.Metadata,
+                DateTime.UtcNow.Add(options.RequestTimeout),
+                cancellationToken);
+
+            if (response.Status?.BSuccess ?? false)
+            {
+                // Measured: server reachable, storage console reporting normally → not-storing baseline.
+                return NotStoring(errorOccurred: false, error: null);
+            }
+
+            byte[] err = response.Status?.BtError?.ToByteArray() ?? [];
+            string detail = err.Length == 0 ? "GetHistorianConsoleStatus returned failure." : Convert.ToHexString(err);
+            return NotStoring(errorOccurred: true, error: $"GetHistorianConsoleStatus failed: {detail}");
+        }
+        catch (OperationCanceledException)
+        {
+            throw;
+        }
+        catch (Exception ex)
+        {
+            // Server unreachable / auth failed — genuinely measured: report it instead of a silent all-false.
+            return NotStoring(errorOccurred: true, error: $"{ex.GetType().Name}: {ex.Message}");
+        }
+    }
+
     /// <summary>
     /// Reads the Historian server's system time-zone name (roadmap item R1.3,
     /// <c>StatusService.GetSystemTimeZoneName</c>). Unlike the 2020 WCF surface — where the native