refactor(engine): extract AutoNester and reorganize NestEngine

Move NFP-based AutoNest logic (polygon extraction, rotation computation,
simulated annealing) into dedicated AutoNester class. Consolidate duplicate
FillWithPairs overloads, extract BuildCandidateAngles and BuildProgressSummary,
reorganize NestEngine into logical sections. Update callers in Console,
MCP tools, and MainForm to use AutoNester.Nest.

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

OpenNest/Forms/MainForm.cs

@@ -762,7 +762,7 @@ namespace OpenNest.Forms
                         activeForm.LoadLastPlate();
 
                     var parts = await Task.Run(() =>
-                        NestEngine.AutoNest(remaining, plate, token));
+                        AutoNester.Nest(remaining, plate, token));
 
                     if (parts.Count == 0)
                         break;
@@ -862,7 +862,9 @@ namespace OpenNest.Forms
             var progress = new Progress<NestProgress>(p =>
             {
                 progressForm.UpdateProgress(p);
-                activeForm.PlateView.SetTemporaryParts(p.BestParts);
+
+                if (p.BestParts != null)
+                    activeForm.PlateView.SetTemporaryParts(p.BestParts);
             });
 
             progressForm.Show(this);
@@ -922,7 +924,9 @@ namespace OpenNest.Forms
             var progress = new Progress<NestProgress>(p =>
             {
                 progressForm.UpdateProgress(p);
-                activeForm.PlateView.SetTemporaryParts(p.BestParts);
+
+                if (p.BestParts != null)
+                    activeForm.PlateView.SetTemporaryParts(p.BestParts);
             });
 
             Action<List<Part>> onComplete = parts =>