test(spike): event-session reuse spike harness (env-gated, B0b)

Opens one v8 Event session and measures SendEvent reuse (register-once, send-many)
+ best-effort read-after-send + optional idle sweep. Skips offline; run live in B0c
to gate event amortization (pending.md A1 broadening, Stage B0).

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

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

@@ -36,6 +36,7 @@ public class EventOnSessionSeamTests
         MethodInfo m = RequireMethod(typeof(HistorianGrpcEventWriteOrchestrator), "OpenAndRegisterEventSession");
         ParameterInfo[] ps = m.GetParameters();
         Assert.Equal("HistorianGrpcConnection", ps[0].ParameterType.Name);
+        Assert.Equal("CancellationToken", ps[1].ParameterType.Name);
         Assert.Equal("Session", m.ReturnType.Name);
     }