feat: add Plate.BuildPerimeterCache with ShapeProfile and convex hull fallback

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

OpenNest.Core/Plate.cs

@@ -125,6 +125,58 @@ namespace OpenNest
             }
         }
 
+        /// <summary>
+        /// Builds a dictionary mapping each non-cut-off part to its perimeter entity.
+        /// Closed shapes use ShapeProfile; open contours fall back to ConvexHull.
+        /// </summary>
+        public static Dictionary<Part, Geometry.Entity> BuildPerimeterCache(Plate plate)
+        {
+            var cache = new Dictionary<Part, Geometry.Entity>();
+
+            foreach (var part in plate.Parts)
+            {
+                if (part.BaseDrawing.IsCutOff)
+                    continue;
+
+                Geometry.Entity perimeter = null;
+                try
+                {
+                    var entities = Converters.ConvertProgram.ToGeometry(part.Program)
+                        .Where(e => e.Layer != SpecialLayers.Rapid)
+                        .ToList();
+
+                    if (entities.Count > 0)
+                    {
+                        var profile = new Geometry.ShapeProfile(entities);
+
+                        if (profile.Perimeter.IsClosed())
+                        {
+                            perimeter = profile.Perimeter;
+                            perimeter.Offset(part.Location);
+                        }
+                        else
+                        {
+                            var points = entities.CollectPoints();
+                            if (points.Count >= 3)
+                            {
+                                var hull = Geometry.ConvexHull.Compute(points);
+                                hull.Offset(part.Location);
+                                perimeter = hull;
+                            }
+                        }
+                    }
+                }
+                catch
+                {
+                    perimeter = null;
+                }
+
+                cache[part] = perimeter;
+            }
+
+            return cache;
+        }
+
         /// <summary>
         /// The number of times to cut the plate.
         /// </summary>

OpenNest.Tests/CutOffGeometryTests.cs

@@ -336,6 +336,48 @@ public class CutOffGeometryTests
         Assert.Equal(8, points.Count);
     }
 
+    [Fact]
+    public void PlatePerimeterCache_ReturnsOneEntryPerPart()
+    {
+        var plate = new Plate(100, 100);
+        plate.Parts.Add(new Part(new Drawing("a", MakeSquare(10))));
+        plate.Parts.Add(new Part(new Drawing("b", MakeCircle(5))));
+        plate.Parts.Add(new Part(new Drawing("c", MakeDiamond(8))));
+
+        var cache = Plate.BuildPerimeterCache(plate);
+        Assert.Equal(3, cache.Count);
+    }
+
+    [Fact]
+    public void PlatePerimeterCache_SkipsCutOffParts()
+    {
+        var plate = new Plate(100, 100);
+        plate.Parts.Add(new Part(new Drawing("real", MakeSquare(10))));
+        plate.Parts.Add(new Part(new Drawing("cutoff", new Program()) { IsCutOff = true }));
+
+        var cache = Plate.BuildPerimeterCache(plate);
+        Assert.Single(cache);
+    }
+
+    [Fact]
+    public void PlatePerimeterCache_OpenContourUsesConvexHull()
+    {
+        var pgm = new Program();
+        pgm.Codes.Add(new RapidMove(new Vector(0, 0)));
+        pgm.Codes.Add(new LinearMove(new Vector(10, 0)));
+        pgm.Codes.Add(new LinearMove(new Vector(10, 10)));
+
+        var plate = new Plate(100, 100);
+        plate.Parts.Add(new Part(new Drawing("open", pgm)));
+
+        var cache = Plate.BuildPerimeterCache(plate);
+        Assert.Single(cache);
+
+        var part = plate.Parts[0];
+        Assert.True(cache.ContainsKey(part));
+        Assert.NotNull(cache[part]);
+    }
+
     [Fact]
     public void ShapeProfile_SelectsLargestShapeAsPerimeter()
     {