refactor: extract responsibilities from NestEngine into focused classes

- Move BuildPairParts to BestFitResult.BuildParts() instance method
- Extract BinConverter (RectanglePacking) for Part/NestItem/Bin conversions
- Extract RotationAnalysis for FindBestRotation and FindHullEdgeAngles

NestEngine reduced from 484 to 287 lines — now purely orchestration,
strategy selection, and comparison logic.

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

OpenNest/Forms/BestFitViewerForm.cs

@@ -126,7 +126,7 @@ namespace OpenNest.Forms
                 result.BoundingWidth,
                 result.BoundingHeight);
 
-            var parts = NestEngine.BuildPairParts(result, drawing);
+            var parts = result.BuildParts(drawing);
 
             foreach (var part in parts)
                 view.Plate.Parts.Add(part);

OpenNest/Forms/MainForm.cs

@@ -537,7 +537,7 @@ namespace OpenNest.Forms
             {
                 if (form.ShowDialog(this) == DialogResult.OK && form.SelectedResult != null)
                 {
-                    var parts = NestEngine.BuildPairParts(form.SelectedResult, drawing);
+                    var parts = form.SelectedResult.BuildParts(drawing);
                     activeForm.PlateView.SetAction(typeof(ActionClone), parts);
                 }
             }