Phase 3B: Site I/O & Observability — Communication, DCL, Script/Alarm actors, Health, Event Logging

Communication Layer (WP-1–5):
- 8 message patterns with correlation IDs, per-pattern timeouts
- Central/Site communication actors, transport heartbeat config
- Connection failure handling (no central buffering, debug streams killed)

Data Connection Layer (WP-6–14, WP-34):
- Connection actor with Become/Stash lifecycle (Connecting/Connected/Reconnecting)
- OPC UA + LmxProxy adapters behind IDataConnection
- Auto-reconnect, bad quality propagation, transparent re-subscribe
- Write-back, tag path resolution with retry, health reporting
- Protocol extensibility via DataConnectionFactory

Site Runtime (WP-15–25, WP-32–33):
- ScriptActor/ScriptExecutionActor (triggers, concurrent execution, blocking I/O dispatcher)
- AlarmActor/AlarmExecutionActor (ValueMatch/RangeViolation/RateOfChange, in-memory state)
- SharedScriptLibrary (inline execution), ScriptRuntimeContext (API)
- ScriptCompilationService (Roslyn, forbidden API enforcement, execution timeout)
- Recursion limit (default 10), call direction enforcement
- SiteStreamManager (per-subscriber bounded buffers, fire-and-forget)
- Debug view backend (snapshot + stream), concurrency serialization
- Local artifact storage (4 SQLite tables)

Health Monitoring (WP-26–28):
- SiteHealthCollector (thread-safe counters, connection state)
- HealthReportSender (30s interval, monotonic sequence numbers)
- CentralHealthAggregator (offline detection 60s, online recovery)

Site Event Logging (WP-29–31):
- SiteEventLogger (SQLite, 6 event categories, ISO 8601 UTC)
- EventLogPurgeService (30-day retention, 1GB cap)
- EventLogQueryService (filters, keyword search, keyset pagination)

541 tests pass, zero warnings.

tests/ScadaLink.Communication.Tests/CentralCommunicationActorTests.cs

@@ -0,0 +1,105 @@
+using Akka.Actor;
+using Akka.TestKit.Xunit2;
+using ScadaLink.Commons.Messages.Communication;
+using ScadaLink.Commons.Messages.Deployment;
+using ScadaLink.Commons.Messages.DebugView;
+using ScadaLink.Commons.Messages.Health;
+using ScadaLink.Communication.Actors;
+
+namespace ScadaLink.Communication.Tests;
+
+/// <summary>
+/// WP-4: Tests for CentralCommunicationActor message routing.
+/// WP-5: Tests for connection failure and failover handling.
+/// </summary>
+public class CentralCommunicationActorTests : TestKit
+{
+    public CentralCommunicationActorTests()
+        : base(@"akka.loglevel = DEBUG")
+    {
+    }
+
+    [Fact]
+    public void RegisterSite_AllowsMessageRouting()
+    {
+        var centralActor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor()));
+
+        // Register a site pointing to the test probe
+        var probe = CreateTestProbe();
+        centralActor.Tell(new RegisterSite("site1", probe.Ref.Path.ToString()));
+
+        // Send a message to the site
+        var command = new DeployInstanceCommand(
+            "dep1", "inst1", "hash1", "{}", "admin", DateTimeOffset.UtcNow);
+        centralActor.Tell(new SiteEnvelope("site1", command));
+
+        // The probe should receive the inner message (not the envelope)
+        probe.ExpectMsg<DeployInstanceCommand>(msg => msg.DeploymentId == "dep1");
+    }
+
+    [Fact]
+    public void UnregisteredSite_MessageIsDropped()
+    {
+        var centralActor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor()));
+
+        var command = new DeployInstanceCommand(
+            "dep1", "inst1", "hash1", "{}", "admin", DateTimeOffset.UtcNow);
+        centralActor.Tell(new SiteEnvelope("unknown-site", command));
+
+        // No crash, no response — the ask will timeout on the caller side
+        ExpectNoMsg(TimeSpan.FromMilliseconds(200));
+    }
+
+    [Fact]
+    public void ConnectionLost_DebugStreamsKilled()
+    {
+        var centralActor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor()));
+        var siteProbe = CreateTestProbe();
+
+        // Register site
+        centralActor.Tell(new RegisterSite("site1", siteProbe.Ref.Path.ToString()));
+
+        // Subscribe to debug view (this tracks the subscription)
+        var subscriberProbe = CreateTestProbe();
+        var subRequest = new SubscribeDebugViewRequest("inst1", "corr-123");
+        centralActor.Tell(new SiteEnvelope("site1", subRequest), subscriberProbe.Ref);
+
+        // Simulate site disconnection
+        centralActor.Tell(new ConnectionStateChanged("site1", false, DateTimeOffset.UtcNow));
+
+        // The subscriber should receive a DebugStreamTerminated notification
+        subscriberProbe.ExpectMsg<DebugStreamTerminated>(
+            msg => msg.SiteId == "site1" && msg.CorrelationId == "corr-123");
+    }
+
+    [Fact]
+    public void ConnectionLost_SiteSelectionRemoved()
+    {
+        var centralActor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor()));
+        var siteProbe = CreateTestProbe();
+
+        centralActor.Tell(new RegisterSite("site1", siteProbe.Ref.Path.ToString()));
+
+        // Disconnect
+        centralActor.Tell(new ConnectionStateChanged("site1", false, DateTimeOffset.UtcNow));
+
+        // Sending a message to the disconnected site should be dropped
+        centralActor.Tell(new SiteEnvelope("site1",
+            new DeployInstanceCommand("dep2", "inst2", "hash2", "{}", "admin", DateTimeOffset.UtcNow)));
+
+        siteProbe.ExpectNoMsg(TimeSpan.FromMilliseconds(200));
+    }
+
+    [Fact]
+    public void Heartbeat_ForwardedToParent()
+    {
+        var parentProbe = CreateTestProbe();
+        var centralActor = parentProbe.ChildActorOf(
+            Props.Create(() => new CentralCommunicationActor()));
+
+        var heartbeat = new HeartbeatMessage("site1", "host1", true, DateTimeOffset.UtcNow);
+        centralActor.Tell(heartbeat);
+
+        parentProbe.ExpectMsg<HeartbeatMessage>(msg => msg.SiteId == "site1");
+    }
+}