perf(dashboard): gate event mirror on live viewers — no clone, no send for unwatched sessions

DashboardEventBroadcaster.Publish ran a deep protobuf Clone (redaction is on by
default) and a group SendAsync for every event of every session, before anything
checked whether a dashboard client was actually watching. In the steady state the
session:{id} group is empty, so that work was thrown away per event.

SignalR does not expose group membership, so EventsHub now mirrors its own
add/remove into a singleton EventsHubViewerRegistry, and OnDisconnectedAsync
releases everything a dropped connection held (SessionDetailsPage disposes the
connection rather than unsubscribing). Publish returns early when the session has
no viewers, before the redaction clone. Watched sessions behave exactly as before.

Lazy mirror-lease start/stop was deliberately not attempted — it entangles the
dashboard with SessionEventDistributor subscribe lifetime for no saving beyond
this gate; recorded in docs/GatewayDashboardDesign.md.

src/ZB.MOM.WW.MxGateway.Server/Dashboard/DashboardServiceCollectionExtensions.cs

@@ -47,6 +47,9 @@ public static class DashboardServiceCollectionExtensions
         services.AddSingleton<HubTokenService>();
         services.AddScoped<Hubs.DashboardHubConnectionFactory>();
         services.AddScoped<IDashboardBrowseService, DashboardBrowseService>();
+        // Singleton: EventsHub instances are transient (one per hub invocation), so the
+        // subscriber bookkeeping they share with the broadcaster must outlive them.
+        services.AddSingleton<Hubs.EventsHubViewerRegistry>();
         services.AddSingleton<Hubs.IDashboardEventBroadcaster, Hubs.DashboardEventBroadcaster>();
         services.AddHostedService<Hubs.DashboardSnapshotPublisher>();
         services.AddHostedService<Hubs.AlarmsHubPublisher>();