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).

code-reviews/Communication/findings.md

@@ -908,8 +908,8 @@ caller, so the reply skips the coordinator.)
 |--|--|
 | Severity | Low |
 | Category | Design-document adherence |
-| Status | Open |
-| Location | `src/ScadaLink.Communication/Actors/SiteCommunicationActor.cs:357-371` |
+| Status | Resolved |
+| Location | `src/ScadaLink.Communication/Actors/SiteCommunicationActor.cs:376-465` |
 
 **Description**
 
@@ -939,6 +939,16 @@ consume `IsActive`; or (b) drop the `IsActive` field from `HeartbeatMessage`
 (additive-only-evolution: deprecate the field, default to `true`, plan
 removal in a major message contract revision).
 
+**Resolution (2026-05-28):** Took option (a). `SiteCommunicationActor` now
+accepts an optional `Func<bool>? isActiveCheck` (default = real `Cluster.Get`
+leader check mirroring `ActiveNodeGate` / `ActiveNodeHealthCheck`) and
+`SendHeartbeatToCentral` stamps `HeartbeatMessage.IsActive` with the result.
+A try/catch keeps the heartbeat tick alive when the cluster state is
+unreadable (warm-up / TestKit without cluster plugin) — falls back to
+`IsActive: false`, the safe non-claiming value. Added parameterised test
+`Heartbeat_StampsIsActive_FromInjectedCheck`. Tests green (199/199 in
+Communication.Tests).
+
 ---
 
 ### Communication-019 — `LoadSiteAddressesFromDb` does not pass a `CancellationToken` to the repository