perf(ui): shared KPI cache, live-cache-backed alarm summary, coalesced debug renders

src/ZB.MOM.WW.ScadaBridge.CentralUI/ServiceCollectionExtensions.cs

@@ -99,7 +99,17 @@ public static class ServiceCollectionExtensions
         // facade over CommunicationService.RequestDebugSnapshotAsync (the same
         // single-shot Ask the Debug View uses) — and flattens the alarm states.
         services.AddScoped<IInstanceSnapshotClient, CommunicationInstanceSnapshotClient>();
-        services.AddScoped<IAlarmSummaryService, AlarmSummaryService>();
+        // The fan-out itself stays scoped (it holds scoped repositories + the snapshot
+        // client), but the page resolves it through SharedAlarmSummaryService — a process
+        // singleton that memoizes the fan-out per site with single-flight, so N circuits
+        // watching one site cost ONE fan-out per window instead of N (arch-review WP2.4).
+        services.AddScoped<AlarmSummaryService>();
+        services.AddSingleton<IAlarmSummaryService, SharedAlarmSummaryService>();
+
+        // Process-level memoized point-in-time KPI snapshots (arch-review WP2.4, finding
+        // #8's per-circuit half). Singleton on purpose: the memo has to outlive any one
+        // circuit for N dashboards to share a single SQL round per refresh window.
+        services.AddSingleton<IKpiSnapshotCache, KpiSnapshotCache>();
 
         // Secured Writes: dispatches the two-person secured-write commands
         // (submit / approve / reject / list) to the central ManagementActor through the