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.Tests/Bending/SolidWorksBendDetectorTests.cs

@@ -0,0 +1,30 @@
+using OpenNest.Bending;
+using OpenNest.IO.Bending;
+
+namespace OpenNest.Tests.Bending;
+
+public class SolidWorksBendDetectorTests
+{
+    [Fact]
+    public void SolidWorksDetector_IsRegistered()
+    {
+        var detector = BendDetectorRegistry.GetByName("SolidWorks");
+        Assert.NotNull(detector);
+        Assert.Equal("SolidWorks", detector.Name);
+    }
+
+    [Fact]
+    public void Registry_ContainsSolidWorksDetector()
+    {
+        Assert.Contains(BendDetectorRegistry.Detectors,
+            d => d.Name == "SolidWorks");
+    }
+
+    [Fact]
+    public void AutoDetect_EmptyDocument_ReturnsEmptyList()
+    {
+        var doc = new ACadSharp.CadDocument();
+        var bends = BendDetectorRegistry.AutoDetect(doc);
+        Assert.Empty(bends);
+    }
+}