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/ClusterInfrastructure/findings.md

@@ -733,8 +733,8 @@ that proves the section name flows from this module to the Host._
 |--|--|
 | Severity | Low |
 | Category | Design-document adherence |
-| Status | Open |
-| Location | `src/ScadaLink.ClusterInfrastructure/ClusterOptionsValidator.cs:30-33` |
+| Status | Resolved |
+| Location | `src/ScadaLink.ClusterInfrastructure/ClusterOptionsValidator.cs:30-43` |
 
 **Description**
 
@@ -775,9 +775,15 @@ list, and add a test case for `SeedNodes.Count == 1` failing validation. Once th
 module's validator enforces the rule, `Host.StartupValidator`'s duplicate check
 becomes redundant and can be removed in the Host's review.
 
-**Resolution**
-
-_Open._
+**Resolution (2026-05-28):** Tightened the seed-node check to require
+`SeedNodes.Count >= 2` with a message that cites Component-ClusterInfrastructure.md
+→ Node Configuration ("both nodes are seed nodes"). Added
+`ClusterOptionsValidatorTests.SingleSeedNode_FailsValidation`; the existing
+`ServiceCollectionExtensionsTests.AddClusterInfrastructure_ValidatorRejectsBadOptionsAtResolution`
+still passes because errors accumulate (MinNrOfMembers=2 still fails as before).
+Removing the duplicate check in `Host.StartupValidator` is left for the Host's
+review per the original recommendation. Tests green (19/19 in
+ClusterInfrastructure.Tests).
 
 ### ClusterInfrastructure-013 — Test uses catastrophic config values without an inline-intent comment