feat: HistorianEventSession primitive + OpenEventSessionAsync (v8 Event reuse)

Reusable v8 Event session wrapping the B0a seams (OpenAndRegisterEventSession once +
SendEventOnSession per op) with a GetSystemParameter keepalive; idempotent dispose.
Mirrors HistorianSession (v6 sibling). pending.md A1 broadening, Stage B1.

Claude-Session: https://claude.ai/code/session_012SDSQ3AcaXqPcBtDESBRii

src/AVEVA.Historian.Client/HistorianClient.cs

@@ -382,6 +382,38 @@ public sealed class HistorianClient : IAsyncDisposable
         }, cancellationToken).ConfigureAwait(false);
     }
 
+    /// <summary>
+    /// Opens a reusable v8 EVENT session (ECDH + RegisterCmEventTag ONCE) over the 2023 R2 gRPC
+    /// transport. The caller owns the session and must dispose it. Reusing the session across sends
+    /// amortizes the ECDH+register cost (~10-16×, spike-proven); the server idle-expires it in ~25s,
+    /// so keep it warm (HistorianEventSession.PingAsync) or re-open. For SendEvent amortization only —
+    /// event reads are gated (C2) and not exposed here. RemoteGrpc only.
+    /// </summary>
+    public async Task<HistorianEventSession> OpenEventSessionAsync(CancellationToken cancellationToken = default)
+    {
+        if (_options.Transport != HistorianTransport.RemoteGrpc)
+        {
+            throw new ProtocolEvidenceMissingException(
+                "HistorianEventSession is only supported over the 2023 R2 RemoteGrpc transport.");
+        }
+
+        return await Task.Run(() =>
+        {
+            Grpc.HistorianGrpcConnection connection = Grpc.HistorianGrpcChannelFactory.Create(_options);
+            try
+            {
+                var orch = new Grpc.HistorianGrpcEventWriteOrchestrator(_options);
+                Grpc.HistorianGrpcHandshake.Session session = orch.OpenAndRegisterEventSession(connection, cancellationToken);
+                return new HistorianEventSession(connection, session, _options);
+            }
+            catch
+            {
+                connection.Dispose();   // don't leak the channel if the handshake fails
+                throw;
+            }
+        }, cancellationToken).ConfigureAwait(false);
+    }
+
     public ValueTask DisposeAsync()
     {
         return ValueTask.CompletedTask;