feat: add IBendDetector interface, SolidWorks implementation, and registry

Introduces a pluggable bend detection system in OpenNest.IO.Bending:
- IBendDetector takes CadDocument directly to preserve MText/layer/linetype info
- SolidWorksBendDetector finds lines on BEND layer with CENTER linetype and matches nearby MText notes
- BendDetectorRegistry auto-registers SolidWorks detector and supports AutoDetect

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

OpenNest.IO/Bending/IBendDetector.cs

@@ -0,0 +1,12 @@
+using ACadSharp;
+using OpenNest.Bending;
+using System.Collections.Generic;
+
+namespace OpenNest.IO.Bending
+{
+    public interface IBendDetector
+    {
+        string Name { get; }
+        List<Bend> DetectBends(CadDocument document);
+    }
+}