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/DataConnectionOptions.cs

@@ -1,8 +1,19 @@
 namespace ScadaLink.DataConnectionLayer;
 
+/// <summary>
+/// Configuration options for the Data Connection Layer.
+/// </summary>
 public class DataConnectionOptions
 {
+    /// <summary>Fixed interval between reconnect attempts after disconnect.</summary>
     public TimeSpan ReconnectInterval { get; set; } = TimeSpan.FromSeconds(5);
+
+    /// <summary>Interval for retrying failed tag path resolution.</summary>
     public TimeSpan TagResolutionRetryInterval { get; set; } = TimeSpan.FromSeconds(10);
+
+    /// <summary>Timeout for synchronous write operations to devices.</summary>
     public TimeSpan WriteTimeout { get; set; } = TimeSpan.FromSeconds(30);
+
+    /// <summary>LmxProxy keep-alive interval for gRPC sessions.</summary>
+    public TimeSpan LmxProxyKeepAliveInterval { get; set; } = TimeSpan.FromSeconds(30);
 }