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/Bend.cs

@@ -0,0 +1,37 @@
+using OpenNest.Geometry;
+using OpenNest.Math;
+
+namespace OpenNest.Bending
+{
+    public class Bend
+    {
+        public Vector StartPoint { get; set; }
+        public Vector EndPoint { get; set; }
+        public BendDirection Direction { get; set; }
+        public double? Angle { get; set; }
+        public double? Radius { get; set; }
+        public string NoteText { get; set; }
+
+        public double Length => StartPoint.DistanceTo(EndPoint);
+
+        public double AngleRadians => Angle.HasValue
+            ? OpenNest.Math.Angle.ToRadians(Angle.Value)
+            : 0;
+
+        public Line ToLine() => new Line(StartPoint, EndPoint);
+
+        /// <summary>
+        /// Returns the angle of the bend line itself (not the bend angle).
+        /// Used for grain direction comparison.
+        /// </summary>
+        public double LineAngle => StartPoint.AngleTo(EndPoint);
+
+        public override string ToString()
+        {
+            var dir = Direction.ToString();
+            var angle = Angle?.ToString("0.##") ?? "?";
+            var radius = Radius?.ToString("0.###") ?? "?";
+            return $"{dir} {angle}° R{radius}";
+        }
+    }
+}

OpenNest.Core/Bending/BendDirection.cs

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