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/BendDetectorRegistry.cs

@@ -0,0 +1,40 @@
+using ACadSharp;
+using OpenNest.Bending;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace OpenNest.IO.Bending
+{
+    public static class BendDetectorRegistry
+    {
+        private static readonly List<IBendDetector> detectors = new();
+
+        static BendDetectorRegistry()
+        {
+            Register(new SolidWorksBendDetector());
+        }
+
+        public static void Register(IBendDetector detector)
+        {
+            detectors.Add(detector);
+        }
+
+        public static IReadOnlyList<IBendDetector> Detectors => detectors;
+
+        public static IBendDetector GetByName(string name)
+        {
+            return detectors.FirstOrDefault(d => d.Name == name);
+        }
+
+        public static List<Bend> AutoDetect(CadDocument document)
+        {
+            foreach (var detector in detectors)
+            {
+                var bends = detector.DetectBends(document);
+                if (bends.Count > 0)
+                    return bends;
+            }
+            return new List<Bend>();
+        }
+    }
+}