feat: add Bend domain model and BendDirection enum to OpenNest.Core

Introduces OpenNest.Core/Bending/ with Bend and BendDirection types as
the foundation for bend line detection. Includes 6 passing unit tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest.Core/Bending/BendDirection.cs

@@ -0,0 +1,9 @@
+namespace OpenNest.Bending
+{
+    public enum BendDirection
+    {
+        Unknown,
+        Up,
+        Down
+    }
+}