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.Commons/Messages/DataConnection/DataConnectionHealthReport.cs

@@ -0,0 +1,13 @@
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.Commons.Messages.DataConnection;
+
+/// <summary>
+/// Health metrics for a single data connection, contributed to the site health report.
+/// </summary>
+public record DataConnectionHealthReport(
+    string ConnectionName,
+    ConnectionHealth Status,
+    int TotalSubscribedTags,
+    int ResolvedTags,
+    DateTimeOffset Timestamp);

src/ScadaLink.Commons/Messages/DataConnection/SubscribeTagsRequest.cs

@@ -0,0 +1,11 @@
+namespace ScadaLink.Commons.Messages.DataConnection;
+
+/// <summary>
+/// Request from an Instance Actor to subscribe to tag values through the DCL.
+/// </summary>
+public record SubscribeTagsRequest(
+    string CorrelationId,
+    string InstanceUniqueName,
+    string ConnectionName,
+    IReadOnlyList<string> TagPaths,
+    DateTimeOffset Timestamp);

src/ScadaLink.Commons/Messages/DataConnection/SubscribeTagsResponse.cs

@@ -0,0 +1,11 @@
+namespace ScadaLink.Commons.Messages.DataConnection;
+
+/// <summary>
+/// Response confirming tag subscription registration.
+/// </summary>
+public record SubscribeTagsResponse(
+    string CorrelationId,
+    string InstanceUniqueName,
+    bool Success,
+    string? ErrorMessage,
+    DateTimeOffset Timestamp);

src/ScadaLink.Commons/Messages/DataConnection/TagValueUpdate.cs

@@ -0,0 +1,21 @@
+using ScadaLink.Commons.Interfaces.Protocol;
+
+namespace ScadaLink.Commons.Messages.DataConnection;
+
+/// <summary>
+/// Published by DCL to an Instance Actor when a subscribed tag value changes.
+/// </summary>
+public record TagValueUpdate(
+    string ConnectionName,
+    string TagPath,
+    object? Value,
+    QualityCode Quality,
+    DateTimeOffset Timestamp);
+
+/// <summary>
+/// Published by DCL when connection state changes, causing bulk quality updates.
+/// </summary>
+public record ConnectionQualityChanged(
+    string ConnectionName,
+    QualityCode Quality,
+    DateTimeOffset Timestamp);

src/ScadaLink.Commons/Messages/DataConnection/UnsubscribeTagsRequest.cs

@@ -0,0 +1,10 @@
+namespace ScadaLink.Commons.Messages.DataConnection;
+
+/// <summary>
+/// Request from an Instance Actor to unsubscribe from all its tags when stopping.
+/// </summary>
+public record UnsubscribeTagsRequest(
+    string CorrelationId,
+    string InstanceUniqueName,
+    string ConnectionName,
+    DateTimeOffset Timestamp);

src/ScadaLink.Commons/Messages/DataConnection/WriteTagRequest.cs

@@ -0,0 +1,21 @@
+namespace ScadaLink.Commons.Messages.DataConnection;
+
+/// <summary>
+/// Request to write a value to a device tag through the DCL.
+/// Write failures are returned synchronously to the calling script.
+/// </summary>
+public record WriteTagRequest(
+    string CorrelationId,
+    string ConnectionName,
+    string TagPath,
+    object? Value,
+    DateTimeOffset Timestamp);
+
+/// <summary>
+/// Response for a device tag write operation.
+/// </summary>
+public record WriteTagResponse(
+    string CorrelationId,
+    bool Success,
+    string? ErrorMessage,
+    DateTimeOffset Timestamp);