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.SiteRuntime/Actors/ScriptExecutionActor.cs

@@ -0,0 +1,126 @@
+using Akka.Actor;
+using Microsoft.CodeAnalysis.Scripting;
+using Microsoft.Extensions.Logging;
+using ScadaLink.Commons.Messages.ScriptExecution;
+using ScadaLink.SiteRuntime.Scripts;
+
+namespace ScadaLink.SiteRuntime.Actors;
+
+/// <summary>
+/// WP-15: Script Execution Actor -- short-lived child of Script Actor.
+/// Receives compiled code, params, Instance Actor ref, and call depth.
+/// Runs on a dedicated blocking I/O dispatcher.
+/// Executes the script via Script Runtime API, returns result, then stops.
+///
+/// WP-32: Script failures are logged but do not disable the script.
+/// Supervision: Stop on unhandled exception (parent ScriptActor decides).
+/// </summary>
+public class ScriptExecutionActor : ReceiveActor
+{
+    public ScriptExecutionActor(
+        string scriptName,
+        string instanceName,
+        Script<object?> compiledScript,
+        IReadOnlyDictionary<string, object?>? parameters,
+        int callDepth,
+        IActorRef instanceActor,
+        SharedScriptLibrary sharedScriptLibrary,
+        SiteRuntimeOptions options,
+        IActorRef replyTo,
+        string correlationId,
+        ILogger logger)
+    {
+        // Immediately begin execution
+        var self = Self;
+        var parent = Context.Parent;
+
+        ExecuteScript(
+            scriptName, instanceName, compiledScript, parameters, callDepth,
+            instanceActor, sharedScriptLibrary, options, replyTo, correlationId,
+            self, parent, logger);
+    }
+
+    private static void ExecuteScript(
+        string scriptName,
+        string instanceName,
+        Script<object?> compiledScript,
+        IReadOnlyDictionary<string, object?>? parameters,
+        int callDepth,
+        IActorRef instanceActor,
+        SharedScriptLibrary sharedScriptLibrary,
+        SiteRuntimeOptions options,
+        IActorRef replyTo,
+        string correlationId,
+        IActorRef self,
+        IActorRef parent,
+        ILogger logger)
+    {
+        var timeout = TimeSpan.FromSeconds(options.ScriptExecutionTimeoutSeconds);
+
+        // CTS must be created inside the async lambda so it outlives this method
+        _ = Task.Run(async () =>
+        {
+            using var cts = new CancellationTokenSource(timeout);
+            try
+            {
+                var context = new ScriptRuntimeContext(
+                    instanceActor,
+                    self,
+                    sharedScriptLibrary,
+                    callDepth,
+                    options.MaxScriptCallDepth,
+                    timeout,
+                    instanceName,
+                    logger);
+
+                var globals = new ScriptGlobals
+                {
+                    Instance = context,
+                    Parameters = parameters ?? new Dictionary<string, object?>(),
+                    CancellationToken = cts.Token
+                };
+
+                var state = await compiledScript.RunAsync(globals, cts.Token);
+
+                // Send result to requester if this was an Ask-based call
+                if (!replyTo.IsNobody())
+                {
+                    replyTo.Tell(new ScriptCallResult(correlationId, true, state.ReturnValue, null));
+                }
+
+                // Notify parent of completion
+                parent.Tell(new ScriptActor.ScriptExecutionCompleted(scriptName, true, null));
+            }
+            catch (OperationCanceledException)
+            {
+                var errorMsg = $"Script '{scriptName}' on instance '{instanceName}' timed out after {timeout.TotalSeconds}s";
+                logger.LogWarning(errorMsg);
+
+                if (!replyTo.IsNobody())
+                {
+                    replyTo.Tell(new ScriptCallResult(correlationId, false, null, errorMsg));
+                }
+
+                parent.Tell(new ScriptActor.ScriptExecutionCompleted(scriptName, false, errorMsg));
+            }
+            catch (Exception ex)
+            {
+                // WP-32: Failures logged to site event log; script NOT disabled after failure
+                var errorMsg = $"Script '{scriptName}' on instance '{instanceName}' failed: {ex.Message}";
+                logger.LogError(ex, "Script execution failed: {Script} on {Instance}", scriptName, instanceName);
+
+                if (!replyTo.IsNobody())
+                {
+                    replyTo.Tell(new ScriptCallResult(correlationId, false, null, errorMsg));
+                }
+
+                parent.Tell(new ScriptActor.ScriptExecutionCompleted(scriptName, false, errorMsg));
+            }
+            finally
+            {
+                // Stop self after execution completes
+                self.Tell(PoisonPill.Instance);
+            }
+        });
+    }
+}