feat(domain): Rubro entity + domain exceptions (CAT-001)

src/api/SIGCM2.Domain/Exceptions/RubroCycleDetectedException.cs

@@ -0,0 +1,17 @@
+namespace SIGCM2.Domain.Exceptions;
+
+/// <summary>
+/// Thrown when moving a Rubro to one of its own descendants would create a cycle. → HTTP 400
+/// </summary>
+public sealed class RubroCycleDetectedException : DomainException
+{
+    public int RubroId { get; }
+    public int NuevoParentId { get; }
+
+    public RubroCycleDetectedException(int rubroId, int nuevoParentId)
+        : base($"Mover el rubro '{rubroId}' al padre '{nuevoParentId}' crearía un ciclo en el árbol.")
+    {
+        RubroId = rubroId;
+        NuevoParentId = nuevoParentId;
+    }
+}