fix: improve arc-tangency fitting and add layered engrave/cut passes for GravographIS

GeometrySimplifier/ArcFit now fit arcs that pass exactly through run
endpoints while balancing tangency error between trusted and estimated
directions, fixing arcs that previously bulged or broke tangent
continuity at fillet/compound-curve junctions.

GravographIS post processor gains per-layer (engrave/cut) tool passes
via a new GravographISPostConfig, so ENGRAVE/ETCH-tagged geometry runs
as a separate scribe pass with its own feed/depth and an operator
pause before the cut pass (spring-floated spindle needs a tool swap).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.Tests/GravographIS/NestPolylineExtractorTests.cs

@@ -34,4 +34,45 @@ public class NestPolylineExtractorTests
         Assert.Equal(new Vector(0.25, 47.75), poly[3]);
         Assert.Equal(new Vector(0.25, 46.75), poly[4]);
     }
+
+    [Fact]
+    public void ExtractPartLayered_SplitsContinuousChainAtLayerChange()
+    {
+        // A single continuous chain (no rapid) that switches from Scribe to Cut
+        // must be split into two layer-uniform polylines sharing the seam vertex,
+        // so the post can emit a tool-change pause between engrave and cut.
+        var program = new Program(Mode.Absolute);
+        program.Codes.Add(new LinearMove(1, 0) { Layer = LayerType.Scribe });
+        program.Codes.Add(new LinearMove(2, 0) { Layer = LayerType.Scribe });
+        program.Codes.Add(new LinearMove(2, 1) { Layer = LayerType.Cut });
+        program.Codes.Add(new LinearMove(3, 1) { Layer = LayerType.Cut });
+
+        var drawing = new Drawing("Mixed", program);
+        var part = new Part(drawing, new Vector(0, 0));
+
+        var polylines = new NestPolylineExtractor().ExtractPartLayered(part);
+
+        Assert.Equal(2, polylines.Count);
+
+        Assert.Equal(LayerType.Scribe, polylines[0].Layer);
+        Assert.Equal(new[] { new Vector(0, 0), new Vector(1, 0), new Vector(2, 0) }, polylines[0].Points);
+
+        Assert.Equal(LayerType.Cut, polylines[1].Layer);
+        Assert.Equal(new[] { new Vector(2, 0), new Vector(2, 1), new Vector(3, 1) }, polylines[1].Points);
+    }
+
+    [Fact]
+    public void ExtractPartLayered_UniformChain_IsSinglePolyline()
+    {
+        var program = new Program(Mode.Absolute);
+        program.Codes.Add(new LinearMove(1, 0));
+        program.Codes.Add(new LinearMove(1, 1));
+
+        var part = new Part(new Drawing("Cut", program), new Vector(0, 0));
+
+        var polylines = new NestPolylineExtractor().ExtractPartLayered(part);
+
+        Assert.Single(polylines);
+        Assert.Equal(LayerType.Cut, polylines[0].Layer);
+    }
 }