fix(sitecallaudit): correct stale relay docs and clarify ack switch

src/ScadaLink.Host/Actors/AkkaHostedService.cs

@@ -415,9 +415,11 @@ akka {{
         // and NotificationOutbox patterns. M3's dual-write transaction routes
         // SiteCalls upserts through AuditLogIngestActor's own scope-per-message
         // ISiteCallAuditRepository resolution, so this singleton is not on the
-        // M3 happy-path hot path; it exists so future direct-write callers
-        // (reconciliation puller, central→site Retry/Discard relay, KPI
-        // projector) Ask through a stable cluster proxy without further wiring.
+        // M3 happy-path hot path; it exists so direct-write callers Ask through
+        // a stable cluster proxy without further wiring. The central→site
+        // Retry/Discard relay now lives in this actor (see the
+        // RegisterCentralCommunication wiring below); the reconciliation puller
+        // is the remaining deferred direct-write caller.
         // Like AuditLogIngestActor, the actor takes the root IServiceProvider
         // and creates a fresh scope per message because ISiteCallAuditRepository
         // is a scoped EF Core service.

src/ScadaLink.SiteCallAudit/SiteCallAuditActor.cs

@@ -23,9 +23,10 @@ namespace ScadaLink.SiteCallAudit;
 /// </summary>
 /// <remarks>
 /// <para>
-/// Query, detail and KPIs land in Task 4; reconciliation and the central→site
-/// Retry/Discard relay remain deferred (per CLAUDE.md scope discipline — they
-/// land in a later follow-up).
+/// Query, detail and KPIs (Task 4) and the central→site Retry/Discard relay
+/// (Task 5 — the relay handlers live in this actor) are implemented; only
+/// reconciliation remains deferred (per CLAUDE.md scope discipline — it lands
+/// in a later follow-up).
 /// </para>
 /// <para>
 /// Per CLAUDE.md "audit-write failure NEVER aborts the user-facing action" —
@@ -550,7 +551,12 @@ public class SiteCallAuditActor : ReceiveActor
             SiteCallRelayOutcome.NotParked =>
                 "The operation is no longer parked at the site (already delivered, discarded, or retrying).",
             SiteCallRelayOutcome.OperationFailed => ack.ErrorMessage,
-            _ => ack.ErrorMessage,
+            // SiteUnreachable is never produced from a ParkedOperationActionAck —
+            // unreachable responses are built by UnreachableRetry/UnreachableDiscard
+            // before any ack is classified, so this arm is unreachable by construction.
+            SiteCallRelayOutcome.SiteUnreachable => ack.ErrorMessage,
+            _ => throw new ArgumentOutOfRangeException(
+                nameof(outcome), outcome, "unknown SiteCallRelayOutcome"),
         };
     }