fix(health-monitoring): resolve HealthMonitoring-004,006,010,011,012 — heartbeat-doc accuracy, testable sequence seeding, logged failures, dead-code removal

src/ScadaLink.HealthMonitoring/SiteHealthState.cs

@@ -30,8 +30,9 @@ public sealed record SiteHealthState
     /// Time the most recent signal of any kind (full report OR heartbeat) was
     /// received. Drives offline detection — heartbeats from the standby keep the
     /// site marked online even when the active node is unable to produce a report
-    /// (mid-failover, brief stalls). See the heartbeat scheduler owned by the
-    /// Cluster Infrastructure / SiteCommunicationActor for the actual cadence.
+    /// (mid-failover, brief stalls). Heartbeat cadence is owned by the Cluster
+    /// Infrastructure / SiteCommunicationActor (every
+    /// CommunicationOptions.TransportHeartbeatInterval — 5s by default).
     /// </summary>
     public DateTimeOffset LastHeartbeatAt { get; init; }