feat: add Drawing.IsCutOff flag for cut-off support

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

OpenNest.Core/Drawing.cs

@@ -56,6 +56,8 @@ namespace OpenNest
 
         public Color Color { get; set; }
 
+        public bool IsCutOff { get; set; }
+
         public NestConstraints Constraints { get; set; }
 
         public SourceInfo Source { get; set; }

OpenNest.Tests/CutOffTests.cs

@@ -0,0 +1,13 @@
+using OpenNest.CNC;
+
+namespace OpenNest.Tests;
+
+public class CutOffTests
+{
+    [Fact]
+    public void Drawing_IsCutOff_DefaultsFalse()
+    {
+        var drawing = new Drawing("test", new Program());
+        Assert.False(drawing.IsCutOff);
+    }
+}