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.HealthMonitoring/HealthReportSender.cs

@@ -0,0 +1,69 @@
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using ScadaLink.Commons.Messages.Health;
+
+namespace ScadaLink.HealthMonitoring;
+
+/// <summary>
+/// Periodically collects a SiteHealthReport and sends it to central via Akka remoting.
+/// Sequence numbers are monotonic, starting at 1, and reset on service restart.
+/// </summary>
+public class HealthReportSender : BackgroundService
+{
+    private readonly ISiteHealthCollector _collector;
+    private readonly IHealthReportTransport _transport;
+    private readonly HealthMonitoringOptions _options;
+    private readonly ILogger<HealthReportSender> _logger;
+    private readonly string _siteId;
+    private long _sequenceNumber;
+
+    public HealthReportSender(
+        ISiteHealthCollector collector,
+        IHealthReportTransport transport,
+        IOptions<HealthMonitoringOptions> options,
+        ILogger<HealthReportSender> logger,
+        ISiteIdentityProvider siteIdentityProvider)
+    {
+        _collector = collector;
+        _transport = transport;
+        _options = options.Value;
+        _logger = logger;
+        _siteId = siteIdentityProvider.SiteId;
+    }
+
+    /// <summary>
+    /// Current sequence number (for testing).
+    /// </summary>
+    public long CurrentSequenceNumber => Interlocked.Read(ref _sequenceNumber);
+
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        _logger.LogInformation(
+            "Health report sender starting for site {SiteId}, interval {Interval}s",
+            _siteId, _options.ReportInterval.TotalSeconds);
+
+        using var timer = new PeriodicTimer(_options.ReportInterval);
+
+        while (await timer.WaitForNextTickAsync(stoppingToken).ConfigureAwait(false))
+        {
+            try
+            {
+                var seq = Interlocked.Increment(ref _sequenceNumber);
+                var report = _collector.CollectReport(_siteId);
+
+                // Replace the placeholder sequence number with our monotonic one
+                var reportWithSeq = report with { SequenceNumber = seq };
+
+                _transport.Send(reportWithSeq);
+
+                _logger.LogDebug("Sent health report #{Seq} for site {SiteId}", seq, _siteId);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex, "Failed to send health report for site {SiteId}", _siteId);
+                // Continue sending — don't let a single failure stop reporting
+            }
+        }
+    }
+}