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.

tests/ScadaLink.SiteEventLogging.Tests/EventLogPurgeServiceTests.cs

@@ -0,0 +1,119 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+
+namespace ScadaLink.SiteEventLogging.Tests;
+
+public class EventLogPurgeServiceTests : IDisposable
+{
+    private readonly SiteEventLogger _eventLogger;
+    private readonly string _dbPath;
+    private readonly SiteEventLogOptions _options;
+
+    public EventLogPurgeServiceTests()
+    {
+        _dbPath = Path.Combine(Path.GetTempPath(), $"test_purge_{Guid.NewGuid()}.db");
+        _options = new SiteEventLogOptions
+        {
+            DatabasePath = _dbPath,
+            RetentionDays = 30,
+            MaxStorageMb = 1024
+        };
+        _eventLogger = new SiteEventLogger(
+            Options.Create(_options),
+            NullLogger<SiteEventLogger>.Instance);
+    }
+
+    public void Dispose()
+    {
+        _eventLogger.Dispose();
+        if (File.Exists(_dbPath)) File.Delete(_dbPath);
+    }
+
+    private EventLogPurgeService CreatePurgeService(SiteEventLogOptions? optionsOverride = null)
+    {
+        var opts = optionsOverride ?? _options;
+        return new EventLogPurgeService(
+            _eventLogger,
+            Options.Create(opts),
+            NullLogger<EventLogPurgeService>.Instance);
+    }
+
+    private void InsertEventWithTimestamp(DateTimeOffset timestamp)
+    {
+        using var cmd = _eventLogger.Connection.CreateCommand();
+        cmd.CommandText = """
+            INSERT INTO site_events (timestamp, event_type, severity, source, message)
+            VALUES ($ts, 'script', 'Info', 'Test', 'Test message')
+            """;
+        cmd.Parameters.AddWithValue("$ts", timestamp.ToString("o"));
+        cmd.ExecuteNonQuery();
+    }
+
+    private long GetEventCount()
+    {
+        using var cmd = _eventLogger.Connection.CreateCommand();
+        cmd.CommandText = "SELECT COUNT(*) FROM site_events";
+        return (long)cmd.ExecuteScalar()!;
+    }
+
+    [Fact]
+    public void PurgeByRetention_DeletesOldEvents()
+    {
+        // Insert an old event (31 days ago) and a recent one
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow.AddDays(-31));
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow);
+
+        var purge = CreatePurgeService();
+        purge.RunPurge();
+
+        Assert.Equal(1, GetEventCount());
+    }
+
+    [Fact]
+    public void PurgeByRetention_KeepsRecentEvents()
+    {
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow.AddDays(-29));
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow.AddDays(-1));
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow);
+
+        var purge = CreatePurgeService();
+        purge.RunPurge();
+
+        Assert.Equal(3, GetEventCount());
+    }
+
+    [Fact]
+    public void PurgeByStorageCap_DeletesOldestWhenOverCap()
+    {
+        // Insert enough events to have some data
+        for (int i = 0; i < 100; i++)
+        {
+            InsertEventWithTimestamp(DateTimeOffset.UtcNow);
+        }
+
+        // Set an artificially small cap to trigger purge
+        var smallCapOptions = new SiteEventLogOptions
+        {
+            DatabasePath = _dbPath,
+            RetentionDays = 30,
+            MaxStorageMb = 0  // 0 MB cap forces purge
+        };
+
+        var purge = CreatePurgeService(smallCapOptions);
+        purge.RunPurge();
+
+        // All events should be purged since cap is 0
+        Assert.Equal(0, GetEventCount());
+    }
+
+    [Fact]
+    public void GetDatabaseSizeBytes_ReturnsPositiveValue()
+    {
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow);
+
+        var purge = CreatePurgeService();
+        var size = purge.GetDatabaseSizeBytes();
+
+        Assert.True(size > 0);
+    }
+}