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.ClusterInfrastructure.Tests/ClusterOptionsValidatorTests.cs

@@ -69,6 +69,23 @@ public class ClusterOptionsValidatorTests
         Assert.Contains("SeedNodes", result.FailureMessage);
     }
 
+    [Fact]
+    public void SingleSeedNode_FailsValidation()
+    {
+        // CI-012: design doc says "both nodes are seed nodes" — a single-seed
+        // configuration defeats the no-startup-ordering-dependency guarantee and
+        // must be rejected by the contract owner's validator, not just by the
+        // Host's startup validator.
+        var options = ValidOptions();
+        options.SeedNodes = new List<string> { "akka.tcp://scadalink@node1:8081" };
+
+        var result = new ClusterOptionsValidator().Validate(null, options);
+
+        Assert.True(result.Failed);
+        Assert.Contains("SeedNodes", result.FailureMessage);
+        Assert.Contains("2", result.FailureMessage);
+    }
+
     [Fact]
     public void HeartbeatNotBelowFailureThreshold_FailsValidation()
     {