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/Hubs/DashboardEventBroadcaster.cs

@@ -21,8 +21,15 @@ namespace ZB.MOM.WW.MxGateway.Server.Dashboard.Hubs;
 /// the mirror independently of the still-outstanding per-session hub ACL
 /// (see <see cref="EventsHub"/>).
 /// </remarks>
+/// <param name="hubContext">Hub context used to send to the session's group.</param>
+/// <param name="viewerRegistry">
+/// Live-subscriber registry consulted before any per-event work is done.
+/// </param>
+/// <param name="options">Gateway options supplying <c>Dashboard:ShowTagValues</c>.</param>
+/// <param name="logger">Logger for best-effort mirror failures.</param>
 public sealed class DashboardEventBroadcaster(
     IHubContext<EventsHub> hubContext,
+    EventsHubViewerRegistry viewerRegistry,
     IOptions<GatewayOptions> options,
     ILogger<DashboardEventBroadcaster> logger) : IDashboardEventBroadcaster
 {
@@ -36,6 +43,16 @@ public sealed class DashboardEventBroadcaster(
             return;
         }
 
+        // Every session's dashboard-mirror subscriber calls Publish for every event,
+        // whether or not a browser is on that session's page. Without this gate the
+        // steady state — no dashboard viewer at all — still paid a deep protobuf
+        // clone (redaction is on by default) plus a send to an empty SignalR group
+        // per event. Bail before both.
+        if (!viewerRegistry.HasViewers(sessionId))
+        {
+            return;
+        }
+
         MxEvent outbound = _showTagValues ? mxEvent : RedactValues(mxEvent);
 
         // Wrap the Task acquisition in a try/catch so a hypothetical synchronous throw