feat: add ActionCutOff for placing sheet cut-offs

Adds a new action that lets users place cut-off lines on plates by
clicking. The action auto-detects horizontal vs vertical axis based on
mouse proximity to plate edges, shows a dashed preview line that
follows the cursor, and adds the cut-off on left click. Pressing
Escape returns to selection mode.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest/Actions/ActionCutOff.cs

@@ -0,0 +1,122 @@
+using OpenNest.CNC;
+using OpenNest.Controls;
+using OpenNest.Geometry;
+using System.ComponentModel;
+using System.Drawing;
+using System.Drawing.Drawing2D;
+using System.Windows.Forms;
+
+namespace OpenNest.Actions
+{
+    [DisplayName("Sheet Cut-Off")]
+    public class ActionCutOff : Action
+    {
+        private CutOff previewCutOff;
+        private CutOffSettings settings;
+
+        public ActionCutOff(PlateView plateView)
+            : base(plateView)
+        {
+            settings = plateView.CutOffSettings;
+
+            plateView.MouseMove += OnMouseMove;
+            plateView.MouseDown += OnMouseDown;
+            plateView.KeyDown += OnKeyDown;
+            plateView.Paint += OnPaint;
+        }
+
+        public override void ConnectEvents()
+        {
+            plateView.MouseMove += OnMouseMove;
+            plateView.MouseDown += OnMouseDown;
+            plateView.KeyDown += OnKeyDown;
+            plateView.Paint += OnPaint;
+        }
+
+        public override void DisconnectEvents()
+        {
+            plateView.MouseMove -= OnMouseMove;
+            plateView.MouseDown -= OnMouseDown;
+            plateView.KeyDown -= OnKeyDown;
+            plateView.Paint -= OnPaint;
+
+            previewCutOff = null;
+            plateView.Invalidate();
+        }
+
+        public override void CancelAction() { }
+
+        public override bool IsBusy() => false;
+
+        private void OnMouseMove(object sender, MouseEventArgs e)
+        {
+            var pt = plateView.CurrentPoint;
+            var axis = DetectAxis(pt);
+
+            previewCutOff = new CutOff(pt, axis);
+            previewCutOff.Regenerate(plateView.Plate, settings);
+            plateView.Invalidate();
+        }
+
+        private void OnMouseDown(object sender, MouseEventArgs e)
+        {
+            if (e.Button != MouseButtons.Left)
+                return;
+
+            var pt = plateView.CurrentPoint;
+            var axis = DetectAxis(pt);
+            var cutoff = new CutOff(pt, axis);
+
+            plateView.Plate.CutOffs.Add(cutoff);
+            plateView.Plate.RegenerateCutOffs(settings);
+            plateView.Invalidate();
+        }
+
+        private void OnKeyDown(object sender, KeyEventArgs e)
+        {
+            if (e.KeyCode == Keys.Escape)
+                plateView.SetAction(typeof(ActionSelect));
+        }
+
+        private void OnPaint(object sender, PaintEventArgs e)
+        {
+            if (previewCutOff?.Drawing?.Program == null)
+                return;
+
+            var program = previewCutOff.Drawing.Program;
+            if (program.Codes.Count == 0)
+                return;
+
+            using var pen = new Pen(Color.FromArgb(128, 64, 64, 64), 1.5f / plateView.ViewScale)
+            {
+                DashStyle = DashStyle.Dash
+            };
+
+            for (var i = 0; i < program.Codes.Count - 1; i += 2)
+            {
+                if (program.Codes[i] is RapidMove rapid &&
+                    program.Codes[i + 1] is LinearMove linear)
+                {
+                    var pt1 = plateView.PointWorldToGraph(rapid.EndPoint);
+                    var pt2 = plateView.PointWorldToGraph(linear.EndPoint);
+                    e.Graphics.DrawLine(pen, pt1, pt2);
+                }
+            }
+        }
+
+        private CutOffAxis DetectAxis(Vector pt)
+        {
+            var bounds = plateView.Plate.BoundingBox(includeParts: false);
+            var distToLeftRight = System.Math.Min(
+                System.Math.Abs(pt.X - bounds.X),
+                System.Math.Abs(pt.X - (bounds.X + bounds.Width)));
+            var distToTopBottom = System.Math.Min(
+                System.Math.Abs(pt.Y - bounds.Y),
+                System.Math.Abs(pt.Y - (bounds.Y + bounds.Length)));
+
+            return distToTopBottom < distToLeftRight
+                ? CutOffAxis.Horizontal
+                : CutOffAxis.Vertical;
+        }
+    }
+}