docs+code: close Theme 1 — 24 design-doc / XML-doc drift findings

Doc/XML-comment drift + small adherence fixes across 17 modules. Highlights:
- Host-017: site CoordinatedShutdown ordering — SiteStreamGrpcServer gains
  CancelAllStreams() (refuse new streams, cancel active), wired into
  Program.cs site branch via ApplicationStopping.
- InboundAPI-021: ParentExecutionId now travels on RouteToGet/SetAttributes
  symmetric with RouteToCallRequest; RouteHelper stamps from _parentExecutionId.
- ClusterInfra-012: ClusterOptionsValidator now requires both seed nodes.
- Comm-018: SiteCommunicationActor.HeartbeatMessage.IsActive derived from
  cluster leader check (was hardcoded true).
- DM-020: reconciliation audit row attributes the current user, not prior deployer.
- SEL-019: EventLogPurgeService early-exits on standby via active-node check.
- Plus comment/XML-doc accuracy fixes across AuditLog, ConfigurationDatabase,
  NotificationOutbox, SiteRuntime, SiteCallAudit; doc refreshes for Component-
  Commons / -ManagementService / -CLI / -ExternalSystemGateway / -HealthMonitoring
  / -Transport / -ConfigurationDatabase; CD-023 index-name doc alignment.

11 new regression tests (RouteHelper x4, SiteStreamGrpcServer x2,
ClusterOptionsValidator x1, SiteCommunicationActor x1, DeploymentService x1,
EventLogPurgeService x3). Build clean (0 warnings); InboundAPI/Communication/
Host suites all green. README regenerated: 112 open (was 136).

tests/ScadaLink.SiteEventLogging.Tests/EventLogPurgeServiceTests.cs

@@ -29,13 +29,16 @@ public class EventLogPurgeServiceTests : IDisposable
         if (File.Exists(_dbPath)) File.Delete(_dbPath);
     }
 
-    private EventLogPurgeService CreatePurgeService(SiteEventLogOptions? optionsOverride = null)
+    private EventLogPurgeService CreatePurgeService(
+        SiteEventLogOptions? optionsOverride = null,
+        SiteEventLogActiveNodeCheck? isActiveNode = null)
     {
         var opts = optionsOverride ?? _options;
         return new EventLogPurgeService(
             _eventLogger,
             Options.Create(opts),
-            NullLogger<EventLogPurgeService>.Instance);
+            NullLogger<EventLogPurgeService>.Instance,
+            isActiveNode);
     }
 
     private void InsertEventWithTimestamp(DateTimeOffset timestamp)
@@ -309,4 +312,54 @@ public class EventLogPurgeServiceTests : IDisposable
 
         Assert.Empty(exceptions);
     }
+
+    // ── SiteEventLogging-019: purge runs only on the active node ──
+
+    [Fact]
+    public void RunPurge_OnStandbyNode_SkipsAllWork()
+    {
+        // SiteEventLogging-019: per design, the daily purge runs on the active
+        // node only. The standby's local SQLite receives no writes, so purging
+        // there is unnecessary; we gate the purge tick on the injected
+        // active-node check and early-exit when it returns false. The row
+        // inserted here is well past retention, so a real purge would delete
+        // it — the standby gate must leave it intact.
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow.AddDays(-31));
+        Assert.Equal(1, GetEventCount());
+
+        var purge = CreatePurgeService(isActiveNode: () => false);
+        purge.RunPurge();
+
+        Assert.Equal(1, GetEventCount());
+    }
+
+    [Fact]
+    public void RunPurge_OnActiveNode_RunsTheRetentionPurge()
+    {
+        // SiteEventLogging-019: when the active-node check returns true the
+        // service runs the purge as before. Pinned alongside the standby case
+        // so a future regression that inverts the gate is caught.
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow.AddDays(-31));
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow);
+
+        var purge = CreatePurgeService(isActiveNode: () => true);
+        purge.RunPurge();
+
+        Assert.Equal(1, GetEventCount());
+    }
+
+    [Fact]
+    public void RunPurge_WithNullCheck_FallsBackToRunning()
+    {
+        // SiteEventLogging-019: when no active-node check is supplied (the
+        // default for non-clustered hosts and pre-existing tests), the service
+        // preserves the pre-fix "run on every tick" behaviour rather than
+        // silently skipping every tick. Backward compatibility guard.
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow.AddDays(-31));
+
+        var purge = CreatePurgeService(isActiveNode: null);
+        purge.RunPurge();
+
+        Assert.Equal(0, GetEventCount());
+    }
 }