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).

src/ScadaLink.Host/Program.cs

@@ -271,6 +271,18 @@ try
         // Map gRPC service — resolves the singleton SiteStreamGrpcServer from DI
         app.MapGrpcService<ScadaLink.Communication.Grpc.SiteStreamGrpcServer>();
 
+        // Host-017 / REQ-HOST-7: site-shutdown ordering. ApplicationStopping
+        // fires BEFORE IHostedService.StopAsync runs, so the gRPC server
+        // refuses new streams (Unavailable) and cancels every active stream
+        // here — clients observe a clean Cancelled and reconnect — and only
+        // THEN does AkkaHostedService run CoordinatedShutdown and tear down
+        // actors. Without this hand-off, in-flight streams go silent and only
+        // time out via gRPC keepalive (~25 s), violating the documented
+        // four-step sequence.
+        var siteLifetime = app.Services.GetRequiredService<Microsoft.Extensions.Hosting.IHostApplicationLifetime>();
+        var siteGrpcServer = app.Services.GetRequiredService<ScadaLink.Communication.Grpc.SiteStreamGrpcServer>();
+        siteLifetime.ApplicationStopping.Register(() => siteGrpcServer.CancelAllStreams());
+
         await app.RunAsync();
     }
     else