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.AuditLog/Central/SiteAuditReconciliationActor.cs

@@ -48,10 +48,13 @@ namespace ScadaLink.AuditLog.Central;
 /// <para>
 /// <b>Failure isolation.</b> A single site that throws (DNS, transport,
 /// repository write) must NOT prevent other sites from being polled on the
-/// same tick. The per-site work runs inside its own try/catch; the actor's
-/// supervisor strategy keeps it alive across any leaked exception with
-/// <see cref="Akka.Actor.SupervisorStrategy.DefaultDecider"/>'s Restart
-/// semantics — restart resets the in-memory cursors, but as noted above that's
+/// same tick. The per-site work runs inside its own try/catch — that
+/// per-site catch is what keeps the actor running across handler throws.
+/// The <see cref="SupervisorStrategy"/> override returns
+/// <see cref="Akka.Actor.SupervisorStrategy.DefaultDecider"/> (Restart
+/// semantics) and governs children only; this actor has no children today,
+/// so the override is a forward-compat placeholder. If it ever did fire,
+/// restart would reset the in-memory cursors — but as noted above that's
 /// a safe (over-pull, idempotent) recovery.
 /// </para>
 /// <para>