feat: add LeadIn hierarchy (7 classes)

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

OpenNest.Core/CNC/CuttingStrategy/LeadIns/LineLeadIn.cs

@@ -0,0 +1,32 @@
+using System.Collections.Generic;
+using OpenNest.Geometry;
+using OpenNest.Math;
+
+namespace OpenNest.CNC.CuttingStrategy
+{
+    public class LineLeadIn : LeadIn
+    {
+        public double Length { get; set; }
+        public double ApproachAngle { get; set; } = 90.0;
+
+        public override List<ICode> Generate(Vector contourStartPoint, double contourNormalAngle,
+            RotationType winding = RotationType.CW)
+        {
+            var piercePoint = GetPiercePoint(contourStartPoint, contourNormalAngle);
+
+            return new List<ICode>
+            {
+                new RapidMove(piercePoint),
+                new LinearMove(contourStartPoint)
+            };
+        }
+
+        public override Vector GetPiercePoint(Vector contourStartPoint, double contourNormalAngle)
+        {
+            var approachAngle = contourNormalAngle + Angle.ToRadians(ApproachAngle);
+            return new Vector(
+                contourStartPoint.X + Length * System.Math.Cos(approachAngle),
+                contourStartPoint.Y + Length * System.Math.Sin(approachAngle));
+        }
+    }
+}