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/Entities/Instances/Instance.cs

@@ -0,0 +1,20 @@
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.Commons.Entities.Instances;
+
+public class Instance
+{
+    public int Id { get; set; }
+    public int TemplateId { get; set; }
+    public int SiteId { get; set; }
+    public int? AreaId { get; set; }
+    public string UniqueName { get; set; }
+    public InstanceState State { get; set; }
+    public ICollection<InstanceAttributeOverride> AttributeOverrides { get; set; } = new List<InstanceAttributeOverride>();
+    public ICollection<InstanceConnectionBinding> ConnectionBindings { get; set; } = new List<InstanceConnectionBinding>();
+
+    public Instance(string uniqueName)
+    {
+        UniqueName = uniqueName ?? throw new ArgumentNullException(nameof(uniqueName));
+    }
+}