refactor: extract SelectionManager from PlateView

Move all selection state and operations (SelectedParts, SelectedCutOffs, DeselectAll, SelectAll, AlignSelected, RotateSelectedParts, PushSelected, GetPartAt*, GetPartsFromWindow, DeleteSelected) into a new internal SelectionManager class. PlateView retains public forwarding methods and properties to preserve the existing API surface. SelectedCutOff property kept public for WinForms designer compatibility.

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

OpenNest.Engine/PlateProcessingResult.cs

@@ -0,0 +1,18 @@
+using OpenNest.CNC;
+using OpenNest.Engine.RapidPlanning;
+using System.Collections.Generic;
+
+namespace OpenNest.Engine
+{
+    public class PlateResult
+    {
+        public List<ProcessedPart> Parts { get; init; }
+    }
+
+    public readonly struct ProcessedPart
+    {
+        public Part Part { get; init; }
+        public Program ProcessedProgram { get; init; }
+        public RapidPath RapidPath { get; init; }
+    }
+}