fix(leads): keep manual lead-ins off a display perimeter

Place Lead-in built its contours from cut-layer entities only, so on a
blank the largest cutout looked like the outside and took an external
lead pierced on the blank; committing then regenerated the part with the
outline as a cut perimeter. The legacy whole-part strategy did the same.

Both strategies now refuse a program with a display perimeter and leave
the part unchanged, and Place Lead-in explains that such a part's cutouts
get their leads from Plan Cutting instead of selecting it.

OpenNest.Core/CNC/CuttingStrategy/ContourCuttingStrategy.cs

@@ -20,6 +20,7 @@ namespace OpenNest.CNC.CuttingStrategy
         public CuttingResult Apply(Program partProgram, Vector approachPoint, Vector nextPartStart)
         {
             var entities = partProgram.ToGeometry();
+            RefuseDisplayPerimeter(entities);
             var breakCuts = TakeBreakCuts(entities);
             entities.RemoveAll(e => e.Layer == SpecialLayers.Rapid);
 
@@ -107,6 +108,14 @@ namespace OpenNest.CNC.CuttingStrategy
             return new CuttingResult { Program = result, LastCutPoint = perimeterPt };
         }
 
+        // These tools would lead the outline or mistake the largest cutout for it (an External lead
+        // pierced on the blank). Only the cutting planner knows every contour of such a part is a hole.
+        private static void RefuseDisplayPerimeter(List<Entity> entities)
+        {
+            if (entities.Exists(e => e.Layer == SpecialLayers.DisplayPerimeter))
+                throw new InvalidOperationException(DisplayPerimeterTail.LeadInRefusal);
+        }
+
         public CuttingResult ApplySingle(
             Program partProgram,
             Vector point,
@@ -115,6 +124,7 @@ namespace OpenNest.CNC.CuttingStrategy
         )
         {
             var entities = partProgram.ToGeometry();
+            RefuseDisplayPerimeter(entities);
             var breakCuts = TakeBreakCuts(entities);
             entities.RemoveAll(e => e.Layer == SpecialLayers.Rapid);
 

OpenNest.Core/CNC/DisplayPerimeterTail.cs

@@ -17,6 +17,11 @@ namespace OpenNest.CNC
         /// <summary>The comment that starts the tail; every code after it is display only.</summary>
         public const string Marker = "DISPLAY PERIMETER NEVER CUT";
 
+        /// <summary>Why the manual and legacy lead-in tools leave a display-perimeter part alone.</summary>
+        public const string LeadInRefusal =
+            "This part's outline is a display perimeter: a supplied blank that is never cut. "
+            + "Lead-ins for its cutouts come from Plan Cutting, which keeps every pierce inside a cutout.";
+
         /// <summary>The index of the tail's marker in <paramref name="program"/>'s codes, or -1.</summary>
         public static int IndexOf(Program program)
         {

OpenNest.Tests/CuttingPlanning/DisplayPerimeterTests.cs

@@ -1,5 +1,6 @@
 using OpenNest.CNC;
 using OpenNest.CNC.CuttingPlanning;
+using OpenNest.Converters;
 using OpenNest.Engine.CuttingPlanning;
 using OpenNest.Geometry;
 
@@ -144,6 +145,25 @@ public class DisplayPerimeterTests
         Assert.DoesNotContain(simulation.Steps, step => step.Layer == LayerType.DisplayPerimeter);
     }
 
+    [Fact]
+    public void ManualAndLegacyLeadIns_RefuseADisplayPerimeterAndLeaveThePartAlone()
+    {
+        var part = new Part(new Drawing("door", Door()), Vector.Zero);
+        var before = ExplicitContourTests.Fingerprint(part.Program);
+        var edge = ConvertProgram.ToGeometry(part.Program).First(e => e.Layer == SpecialLayers.DisplayPerimeter);
+        var hole = ConvertProgram.ToGeometry(part.Program).First(e => e.Layer == SpecialLayers.Cut);
+
+        var onEdge = Assert.Throws<InvalidOperationException>(() => part.ApplySingleLeadIn(
+            ExplicitContourTests.Parameters(), new Vector(0, 10), edge, OpenNest.CNC.CuttingStrategy.ContourType.External));
+        Assert.Throws<InvalidOperationException>(() => part.ApplySingleLeadIn(
+            ExplicitContourTests.Parameters(), new Vector(5, 7), hole, OpenNest.CNC.CuttingStrategy.ContourType.Internal));
+        Assert.Throws<InvalidOperationException>(() => part.ApplyLeadIns(ExplicitContourTests.Parameters(), Vector.Zero));
+
+        Assert.Contains("Plan Cutting", onEdge.Message);
+        Assert.False(part.HasManualLeadIns);
+        Assert.Equal(before, ExplicitContourTests.Fingerprint(part.Program));
+    }
+
     [Fact]
     public void Posting_RefusesAnUnplannedDisplayPerimeter()
     {

OpenNest/Actions/ActionLeadIn.cs

@@ -423,6 +423,14 @@ namespace OpenNest.Actions
             if (part.BaseDrawing.IsCutOff)
                 return;
 
+            // A supplied blank's cutouts get their leads from Plan Cutting only.
+            if (CNC.DisplayPerimeterTail.HasDisplayPerimeterMotion(part.BaseDrawing.Program))
+            {
+                MessageBox.Show(plateView.FindForm(), CNC.DisplayPerimeterTail.LeadInRefusal, "Place Lead-in",
+                    MessageBoxButtons.OK, MessageBoxIcon.Information);
+                return;
+            }
+
             selectedLayoutPart = layoutPart;
             selectedPart = part;