test(communication): close register row 102 — serialize the two gauge-moving classes so the subscribers.active baseline holds

The gauge test polls baseline+1, but the process-global gauge's baseline is
captured once: a parallel class's streams closing after capture lowers the
gauge below baseline+1 forever. Survey confirmed only SiteStreamGrpcServerTests
and CleanupVerificationTests drive the real SubscribeInstance handler that
moves the counter; both now share the SiteStreamSubscriberGauge collection
(row-96 pattern). 4/4 full-suite runs green, durations unchanged.

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/Grpc/CleanupVerificationTests.cs

@@ -11,6 +11,10 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Tests.Grpc;
 /// Verifies that after a gRPC stream is cancelled, the SiteStreamManager
 /// subscription is properly cleaned up with no leaked subscriptions.
 /// </summary>
+// Serialized with SiteStreamGrpcServerTests (register row 102) — this class's real
+// SiteStreamGrpcServer streams move the process-global subscribers.active gauge that
+// SiteStreamSubscribersActiveGauge_GoesToOneOnConnect_AndBackToZeroOnCancel baselines.
+[Collection("SiteStreamSubscriberGauge")]
 public class CleanupVerificationTests : TestKit
 {
     [Fact]

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/Grpc/SiteStreamGrpcServerTests.cs

@@ -11,6 +11,12 @@ using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
 
 namespace ZB.MOM.WW.ScadaBridge.Communication.Tests.Grpc;
 
+// Serialized with CleanupVerificationTests (register row 102): these are the only two
+// classes whose streams move the process-global scadabridge.site.stream.subscribers.active
+// gauge, and the gauge test below captures its baseline ONCE — a parallel class's streams
+// CLOSING after that capture makes baseline+1 unreachable (fixed-baseline deltas only
+// survive contributors that hold or increase). Same collection ⇒ never concurrent.
+[Collection("SiteStreamSubscriberGauge")]
 public class SiteStreamGrpcServerTests : TestKit
 {
     private readonly ISiteStreamSubscriber _subscriber;
@@ -481,9 +487,11 @@ public class SiteStreamGrpcServerTests : TestKit
 
         await WaitForConditionAsync(() => server.ActiveStreamCount == 1);
 
-        // While the stream is up the gauge is one above whatever baseline other
-        // (possibly parallel) tests left behind — read relative so the assertion
-        // is robust to test interleaving on the process-wide static counter.
+        // While the stream is up the gauge is one above the captured baseline. The
+        // relative read alone is NOT enough on a process-wide counter — a parallel
+        // class's streams closing after the capture lowers the gauge below
+        // baseline+1 forever (register row 102) — so the classes that move this
+        // gauge are serialized into the SiteStreamSubscriberGauge collection.
         //
         // SiteStreamSubscriberOpened() runs AFTER the _activeStreams insertion that
         // WaitForConditionAsync(ActiveStreamCount == 1) keys off (see