Resolve all 46 low-severity architecture review findings

src/ZB.MOM.WW.ScadaBridge.Host/Health/RequiredSingletonsHealthCheck.cs

@@ -29,12 +29,13 @@ namespace ZB.MOM.WW.ScadaBridge.Host.Health;
 /// whole check stays cheap and readiness polling stays fast.
 /// </para>
 /// <para>
-/// <b>Required-always vs if-applicable.</b> All five central singleton proxies are
-/// created unconditionally on a central node (there is no feature/config gate around
-/// any of them), so all five are treated as required-always here. If a future
-/// singleton is created behind a feature flag, it should NOT be added to
-/// <see cref="RequiredSingletonProxyNames"/> — "if applicable" means skip when its
-/// feature is off.
+/// <b>Required versus best-effort.</b> Seven central singleton proxies are created
+/// unconditionally. The five listed in <see cref="RequiredSingletonProxyNames"/>
+/// are required for readiness. The KPI-history recorder and pending-deployment purge
+/// are deliberately excluded: they provide best-effort observability and hygiene,
+/// respectively, and their availability is not a correctness prerequisite. Future
+/// additions must be classified by readiness impact, not just unconditional creation;
+/// a feature-gated singleton must also be skipped when its feature is disabled.
 /// </para>
 /// <para>
 /// <b>Failover flakiness.</b> During a brief singleton handover the singleton may be
@@ -60,7 +61,7 @@ public sealed class RequiredSingletonsHealthCheck : IHealthCheck
     /// <summary>
     /// Local actor names (under <c>/user</c>) of the <c>ClusterSingletonProxy</c>
     /// actors for the singletons that must always be running on a central node.
-    /// Matches the unconditional proxy registrations in
+    /// Selects the five required services from the seven unconditional registrations in
     /// <c>AkkaHostedService.RegisterCentralActors</c>.
     /// </summary>
     public static readonly IReadOnlyList<string> RequiredSingletonProxyNames = new[]