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.Tests/Bending/BendModelTests.cs

@@ -0,0 +1,90 @@
+using OpenNest.Bending;
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests.Bending;
+
+public class BendModelTests
+{
+    [Fact]
+    public void Bend_StoresStartAndEndPoints()
+    {
+        var bend = new Bend
+        {
+            StartPoint = new Vector(0, 5),
+            EndPoint = new Vector(10, 5),
+            Direction = BendDirection.Up,
+            Angle = 90,
+            Radius = 0.06,
+            NoteText = "UP 90° R0.06"
+        };
+
+        Assert.Equal(0, bend.StartPoint.X);
+        Assert.Equal(10, bend.EndPoint.X);
+        Assert.Equal(BendDirection.Up, bend.Direction);
+        Assert.Equal(90, bend.Angle);
+        Assert.Equal(0.06, bend.Radius);
+        Assert.Equal("UP 90° R0.06", bend.NoteText);
+    }
+
+    [Fact]
+    public void Bend_ToLine_ReturnsGeometryLine()
+    {
+        var bend = new Bend
+        {
+            StartPoint = new Vector(0, 5),
+            EndPoint = new Vector(10, 5)
+        };
+
+        var line = bend.ToLine();
+
+        Assert.Equal(0, line.StartPoint.X);
+        Assert.Equal(5, line.StartPoint.Y);
+        Assert.Equal(10, line.EndPoint.X);
+        Assert.Equal(5, line.EndPoint.Y);
+    }
+
+    [Fact]
+    public void Bend_Length_ComputesCorrectly()
+    {
+        var bend = new Bend
+        {
+            StartPoint = new Vector(0, 0),
+            EndPoint = new Vector(3, 4)
+        };
+
+        Assert.Equal(5.0, bend.Length, 0.001);
+    }
+
+    [Fact]
+    public void Bend_BendAngleRadians_ConvertsDegreesToRadians()
+    {
+        var bend = new Bend { Angle = 90 };
+
+        Assert.Equal(System.Math.PI / 2, bend.AngleRadians, 0.001);
+    }
+
+    [Fact]
+    public void Bend_DefaultDirection_IsUnknown()
+    {
+        var bend = new Bend();
+
+        Assert.Equal(BendDirection.Unknown, bend.Direction);
+    }
+
+    [Fact]
+    public void Bend_ToString_FormatsNicely()
+    {
+        var bend = new Bend
+        {
+            Direction = BendDirection.Up,
+            Angle = 90,
+            Radius = 0.06
+        };
+
+        var str = bend.ToString();
+
+        Assert.Contains("Up", str);
+        Assert.Contains("90", str);
+        Assert.Contains("0.06", str);
+    }
+}