refactor: update PlateRenderer for SelectionManager cut-off list

PlateRenderer now checks Selection.SelectedCutOffs.Contains() instead
of comparing against a single SelectedCutOff property. Remove temporary
SelectedCutOff shim from PlateView and unused Designer assignment.

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

OpenNest/Controls/PlateRenderer.cs

@@ -168,7 +168,7 @@ namespace OpenNest.Controls
                 if (program == null || program.Codes.Count == 0)
                     continue;
 
-                var activePen = cutoff == view.SelectedCutOff ? selectedPen : pen;
+                var activePen = view.Selection.SelectedCutOffs.Contains(cutoff) ? selectedPen : pen;
 
                 for (var i = 0; i < program.Codes.Count - 1; i += 2)
                 {

OpenNest/Controls/PlateView.cs

@@ -179,18 +179,6 @@ namespace OpenNest.Controls
             }
         }
 
-        // Temporary — removed in Task 5
-        public CutOff SelectedCutOff
-        {
-            get => selection.SelectedCutOffs.Count > 0 ? selection.SelectedCutOffs[0] : null;
-            set
-            {
-                selection.SelectedCutOffs.Clear();
-                if (value != null)
-                    selection.SelectedCutOffs.Add(value);
-                Invalidate();
-            }
-        }
 
         public double RotateIncrementAngle { get; set; }
 

OpenNest/Forms/NestProgressForm.Designer.cs

@@ -427,7 +427,7 @@ namespace OpenNest.Forms
             plate1.Quantity = 0;
             previewPlateView.Plate = plate1;
             previewPlateView.RotateIncrementAngle = 10D;
-            previewPlateView.SelectedCutOff = null;
+
             previewPlateView.ShowBendLines = false;
             previewPlateView.Size = new System.Drawing.Size(356, 341);
             previewPlateView.Status = "Select";