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/DataConnectionFactory.cs

@@ -0,0 +1,43 @@
+using Microsoft.Extensions.Logging;
+using ScadaLink.Commons.Interfaces.Protocol;
+using ScadaLink.DataConnectionLayer.Adapters;
+
+namespace ScadaLink.DataConnectionLayer;
+
+/// <summary>
+/// WP-34: Default factory that resolves protocol type strings to IDataConnection adapters.
+/// Protocol extensibility: register new adapters via the constructor or AddAdapter method.
+/// </summary>
+public class DataConnectionFactory : IDataConnectionFactory
+{
+    private readonly Dictionary<string, Func<IDictionary<string, string>, IDataConnection>> _factories = new(StringComparer.OrdinalIgnoreCase);
+    private readonly ILoggerFactory _loggerFactory;
+
+    public DataConnectionFactory(ILoggerFactory loggerFactory)
+    {
+        _loggerFactory = loggerFactory;
+
+        // Register built-in protocols
+        RegisterAdapter("OpcUa", details => new OpcUaDataConnection(
+            new DefaultOpcUaClientFactory(), _loggerFactory.CreateLogger<OpcUaDataConnection>()));
+        RegisterAdapter("LmxProxy", details => new LmxProxyDataConnection(
+            new DefaultLmxProxyClientFactory(), _loggerFactory.CreateLogger<LmxProxyDataConnection>()));
+    }
+
+    /// <summary>
+    /// Registers a new protocol adapter factory. This is the extension point
+    /// for adding new protocols without modifying existing code.
+    /// </summary>
+    public void RegisterAdapter(string protocolType, Func<IDictionary<string, string>, IDataConnection> factory)
+    {
+        _factories[protocolType] = factory;
+    }
+
+    public IDataConnection Create(string protocolType, IDictionary<string, string> connectionDetails)
+    {
+        if (!_factories.TryGetValue(protocolType, out var factory))
+            throw new ArgumentException($"Unknown protocol type: {protocolType}. Registered protocols: {string.Join(", ", _factories.Keys)}");
+
+        return factory(connectionDetails);
+    }
+}