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.

src/ScadaLink.DataConnectionLayer/Actors/DataConnectionManagerActor.cs

@@ -0,0 +1,142 @@
+using Akka.Actor;
+using Akka.Event;
+using ScadaLink.Commons.Interfaces.Protocol;
+using ScadaLink.Commons.Messages.DataConnection;
+
+namespace ScadaLink.DataConnectionLayer.Actors;
+
+/// <summary>
+/// WP-34: Protocol extensibility — manages DataConnectionActor instances.
+/// Routes messages to the correct connection actor based on connection name.
+/// Adding a new protocol = implement IDataConnection + register with IDataConnectionFactory.
+/// </summary>
+public class DataConnectionManagerActor : ReceiveActor
+{
+    private readonly ILoggingAdapter _log = Context.GetLogger();
+    private readonly IDataConnectionFactory _factory;
+    private readonly DataConnectionOptions _options;
+    private readonly Dictionary<string, IActorRef> _connectionActors = new();
+
+    public DataConnectionManagerActor(
+        IDataConnectionFactory factory,
+        DataConnectionOptions options)
+    {
+        _factory = factory;
+        _options = options;
+
+        Receive<CreateConnectionCommand>(HandleCreateConnection);
+        Receive<SubscribeTagsRequest>(HandleRoute);
+        Receive<UnsubscribeTagsRequest>(HandleRoute);
+        Receive<WriteTagRequest>(HandleRouteWrite);
+        Receive<RemoveConnectionCommand>(HandleRemoveConnection);
+        Receive<GetAllHealthReports>(HandleGetAllHealthReports);
+    }
+
+    private void HandleCreateConnection(CreateConnectionCommand command)
+    {
+        if (_connectionActors.ContainsKey(command.ConnectionName))
+        {
+            _log.Warning("Connection {0} already exists", command.ConnectionName);
+            return;
+        }
+
+        // WP-34: Factory creates the correct adapter based on protocol type
+        var adapter = _factory.Create(command.ProtocolType, command.ConnectionDetails);
+
+        var props = Props.Create(() => new DataConnectionActor(
+            command.ConnectionName, adapter, _options));
+
+        var actorRef = Context.ActorOf(props, command.ConnectionName);
+        _connectionActors[command.ConnectionName] = actorRef;
+
+        _log.Info("Created DataConnectionActor for {0} (protocol={1})",
+            command.ConnectionName, command.ProtocolType);
+    }
+
+    private void HandleRoute(SubscribeTagsRequest request)
+    {
+        if (_connectionActors.TryGetValue(request.ConnectionName, out var actor))
+            actor.Forward(request);
+        else
+        {
+            _log.Warning("No connection actor for {0}", request.ConnectionName);
+            Sender.Tell(new SubscribeTagsResponse(
+                request.CorrelationId, request.InstanceUniqueName, false,
+                $"Unknown connection: {request.ConnectionName}", DateTimeOffset.UtcNow));
+        }
+    }
+
+    private void HandleRoute(UnsubscribeTagsRequest request)
+    {
+        if (_connectionActors.TryGetValue(request.ConnectionName, out var actor))
+            actor.Forward(request);
+        else
+            _log.Warning("No connection actor for {0} during unsubscribe", request.ConnectionName);
+    }
+
+    private void HandleRouteWrite(WriteTagRequest request)
+    {
+        if (_connectionActors.TryGetValue(request.ConnectionName, out var actor))
+            actor.Forward(request);
+        else
+        {
+            _log.Warning("No connection actor for {0}", request.ConnectionName);
+            Sender.Tell(new WriteTagResponse(
+                request.CorrelationId, false,
+                $"Unknown connection: {request.ConnectionName}", DateTimeOffset.UtcNow));
+        }
+    }
+
+    private void HandleRemoveConnection(RemoveConnectionCommand command)
+    {
+        if (_connectionActors.TryGetValue(command.ConnectionName, out var actor))
+        {
+            Context.Stop(actor);
+            _connectionActors.Remove(command.ConnectionName);
+            _log.Info("Removed DataConnectionActor for {0}", command.ConnectionName);
+        }
+    }
+
+    private void HandleGetAllHealthReports(GetAllHealthReports _)
+    {
+        // Forward health report requests to all connection actors
+        foreach (var actor in _connectionActors.Values)
+        {
+            actor.Forward(new DataConnectionActor.GetHealthReport());
+        }
+    }
+
+    /// <summary>
+    /// OneForOneStrategy with Restart for connection actors — a failed connection
+    /// should restart and attempt reconnection.
+    /// </summary>
+    protected override SupervisorStrategy SupervisorStrategy()
+    {
+        return new OneForOneStrategy(
+            maxNrOfRetries: 10,
+            withinTimeRange: TimeSpan.FromMinutes(1),
+            decider: Decider.From(ex =>
+            {
+                _log.Warning(ex, "DataConnectionActor threw exception, restarting");
+                return Directive.Restart;
+            }));
+    }
+}
+
+/// <summary>
+/// Command to create a new data connection actor for a specific protocol.
+/// </summary>
+public record CreateConnectionCommand(
+    string ConnectionName,
+    string ProtocolType,
+    IDictionary<string, string> ConnectionDetails);
+
+/// <summary>
+/// Command to remove a data connection actor.
+/// </summary>
+public record RemoveConnectionCommand(string ConnectionName);
+
+/// <summary>
+/// Request for health reports from all active connections.
+/// </summary>
+public record GetAllHealthReports;