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/Interfaces/Protocol/IDataConnection.cs

@@ -0,0 +1,25 @@
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.Commons.Interfaces.Protocol;
+
+public enum QualityCode { Good, Bad, Uncertain }
+
+public record TagValue(object? Value, QualityCode Quality, DateTimeOffset Timestamp);
+public record ReadResult(bool Success, TagValue? Value, string? ErrorMessage);
+public record WriteResult(bool Success, string? ErrorMessage);
+
+public delegate void SubscriptionCallback(string tagPath, TagValue value);
+
+public interface IDataConnection : IAsyncDisposable
+{
+    Task ConnectAsync(IDictionary<string, string> connectionDetails, CancellationToken cancellationToken = default);
+    Task DisconnectAsync(CancellationToken cancellationToken = default);
+    Task<string> SubscribeAsync(string tagPath, SubscriptionCallback callback, CancellationToken cancellationToken = default);
+    Task UnsubscribeAsync(string subscriptionId, CancellationToken cancellationToken = default);
+    Task<ReadResult> ReadAsync(string tagPath, CancellationToken cancellationToken = default);
+    Task<IReadOnlyDictionary<string, ReadResult>> ReadBatchAsync(IEnumerable<string> tagPaths, CancellationToken cancellationToken = default);
+    Task<WriteResult> WriteAsync(string tagPath, object? value, CancellationToken cancellationToken = default);
+    Task<IReadOnlyDictionary<string, WriteResult>> WriteBatchAsync(IDictionary<string, object?> values, CancellationToken cancellationToken = default);
+    Task<bool> WriteBatchAndWaitAsync(IDictionary<string, object?> values, string flagPath, object? flagValue, string responsePath, object? responseValue, TimeSpan timeout, CancellationToken cancellationToken = default);
+    ConnectionHealth Status { get; }
+}