Phase 0 WP-0.2–0.9: Implement Commons (types, entities, interfaces, messages, protocol, tests)

- WP-0.2: Namespace/folder skeleton (26 directories)
- WP-0.3: Shared data types (6 enums, RetryPolicy, Result<T>)
- WP-0.4: 24 domain entity POCOs across 10 domain areas
- WP-0.5: 7 repository interfaces with full CRUD signatures
- WP-0.6: IAuditService cross-cutting interface
- WP-0.7: 26 message contract records across 8 concern areas
- WP-0.8: IDataConnection protocol abstraction with batch ops
- WP-0.9: 8 architectural constraint enforcement tests
All 40 tests pass, zero warnings.

src/ScadaLink.Commons/Messages/Health/HeartbeatMessage.cs

@@ -0,0 +1,7 @@
+namespace ScadaLink.Commons.Messages.Health;
+
+public record HeartbeatMessage(
+    string SiteId,
+    string NodeHostname,
+    bool IsActive,
+    DateTimeOffset Timestamp);

src/ScadaLink.Commons/Messages/Health/SiteHealthReport.cs

@@ -0,0 +1,14 @@
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.Commons.Messages.Health;
+
+public record SiteHealthReport(
+    string SiteId,
+    long SequenceNumber,
+    DateTimeOffset ReportTimestamp,
+    IReadOnlyDictionary<string, ConnectionHealth> DataConnectionStatuses,
+    IReadOnlyDictionary<string, TagResolutionStatus> TagResolutionCounts,
+    int ScriptErrorCount,
+    int AlarmEvaluationErrorCount,
+    IReadOnlyDictionary<string, int> StoreAndForwardBufferDepths,
+    int DeadLetterCount);

src/ScadaLink.Commons/Messages/Health/TagResolutionStatus.cs

@@ -0,0 +1,3 @@
+namespace ScadaLink.Commons.Messages.Health;
+
+public record TagResolutionStatus(int TotalSubscribed, int SuccessfullyResolved);