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

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

@@ -83,6 +83,31 @@ public sealed class HistorianGrpcIntegrationTests
         Assert.False(string.IsNullOrWhiteSpace(zone));
     }
 
+    [Fact]
+    public async Task GetStoreForwardStatusAsync_OverGrpc_ReturnsMeasuredIdleState()
+    {
+        string? host = Environment.GetEnvironmentVariable("HISTORIAN_GRPC_HOST");
+        if (string.IsNullOrWhiteSpace(host) || string.IsNullOrEmpty(Environment.GetEnvironmentVariable("HISTORIAN_USER")))
+        {
+            return;
+        }
+
+        // R4.3 measured idle-state: over gRPC, GetStoreForwardStatusAsync actually contacts the server
+        // (StatusService.GetHistorianConsoleStatus) rather than synthesizing. On an idle/normal server
+        // it reports the not-storing baseline WITHOUT ErrorOccurred. The active-SF buffer magnitude
+        // lives behind the D2 storage-engine console wall and is intentionally not surfaced (stays
+        // false). See docs/plans/store-forward-cache-reverse-engineering.md §9.7.
+        HistorianClient client = new(BuildOptions(host));
+        HistorianStoreForwardStatus status = await client.GetStoreForwardStatusAsync(CancellationToken.None);
+
+        Assert.Equal(host, status.ServerName);
+        Assert.False(status.ErrorOccurred, $"reachable server should not report an error: {status.Error}");
+        Assert.Null(status.Error);
+        Assert.False(status.Storing);
+        Assert.False(status.Pending);
+        Assert.False(status.DataStored);
+    }
+
     [Fact]
     public async Task GetTagMetadataAsync_OverGrpc_ReturnsRequestedTag()
     {