fix: preserve leading rapid in programs to prevent missing contour segment

The CAD converter and BOM import were stripping the leading RapidMove
after normalizing program coordinates to origin. This left programs
starting with a LinearMove, causing the post-processor to use that
endpoint as the pierce point — making the first contour edge zero-length
and losing the closing segment (e.g. the bottom line on curved parts).

Root cause: CadConverterForm.GetDrawings(), OnSplitClicked(), and
BomImportForm all called pgm.Codes.RemoveAt(0) after offsetting the
rapid to origin. The rapid at (0,0) is a harmless no-op that marks the
contour start point for downstream processing.

Also adds EnsureLeadingRapid() safety net in the Cincinnati post for
existing nest files that already have the rapid stripped.

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

OpenNest.Tests/Cincinnati/CincinnatiPostProcessorTests.cs

@@ -388,6 +388,57 @@ public class CincinnatiPostProcessorTests
         Assert.Equal(300, deserialized.PartSubprogramStart);
     }
 
+    [Fact]
+    public void Post_ClosedContourWithoutRapid_OutputsAllSegments()
+    {
+        // Reproduce bug: a closed contour with no leading rapid loses its
+        // first segment in the CNC output because the feature writer uses
+        // the first LinearMove endpoint as the pierce point.
+        var pgm = new Program(Mode.Incremental);
+        pgm.Codes.Add(new LinearMove(0, 2));    // (0,0) → (0,2)
+        pgm.Codes.Add(new LinearMove(2, 0));    // (0,2) → (2,2)
+        pgm.Codes.Add(new LinearMove(0, -2));   // (2,2) → (2,0)
+        pgm.Codes.Add(new LinearMove(-2, 0));   // (2,0) → (0,0)
+
+        var drawing = new Drawing("ClosedSquare", pgm);
+        var nest = new Nest("TestClosure");
+        nest.Drawings.Add(drawing);
+        var plate = new Plate(24, 24);
+        plate.Parts.Add(new Part(drawing, new Vector(1, 1)));
+        nest.Plates.Add(plate);
+
+        var config = new CincinnatiPostConfig
+        {
+            UsePartSubprograms = true,
+            PostedAccuracy = 4
+        };
+        var post = new CincinnatiPostProcessor(config);
+
+        using var ms = new MemoryStream();
+        post.Post(nest, ms);
+        var output = Encoding.UTF8.GetString(ms.ToArray());
+
+        // The subprogram should contain all 4 segments of the square.
+        // Without the fix, the first segment (0,0)→(0,2) was lost
+        // because the feature writer pierced at (0,2) making it zero-length.
+        var lines = output.Split('\n').Select(l => l.Trim()).ToArray();
+
+        // Count G1 moves in the subprogram (should be 4 for a square)
+        var subStart = Array.FindIndex(lines, l => l.StartsWith(":200") || l.StartsWith(":300"));
+        Assert.True(subStart >= 0, "Expected a part subprogram");
+
+        var g1Count = 0;
+        for (var i = subStart; i < lines.Length; i++)
+        {
+            if (lines[i].StartsWith("M99"))
+                break;
+            if (lines[i].Contains("G1 "))
+                g1Count++;
+        }
+
+        Assert.Equal(4, g1Count);
+    }
+
     private static Nest CreateTestNest()
     {
         var nest = new Nest("TestNest");