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/Converters/ConvertGeometryLayerTests.cs

@@ -0,0 +1,70 @@
+using System.Linq;
+using OpenNest.CNC;
+using OpenNest.Converters;
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests.Converters;
+
+public class ConvertGeometryLayerTests
+{
+    private static Program ProgramFor(Entity entity)
+    {
+        var shape = new Shape();
+        shape.Entities.Add(entity);
+        return ConvertGeometry.ToProgram(shape);
+    }
+
+    [Fact]
+    public void AddLine_EngraveLayer_TagsScribe()
+    {
+        var line = new Line(0, 0, 1, 0) { Layer = new Layer("ENGRAVE") };
+
+        var pgm = ProgramFor(line);
+
+        Assert.All(pgm.Codes.OfType<LinearMove>(), m => Assert.Equal(LayerType.Scribe, m.Layer));
+    }
+
+    [Fact]
+    public void AddLine_EtchLayer_TagsScribe()
+    {
+        var line = new Line(0, 0, 1, 0) { Layer = new Layer("etch") };
+
+        var pgm = ProgramFor(line);
+
+        Assert.All(pgm.Codes.OfType<LinearMove>(), m => Assert.Equal(LayerType.Scribe, m.Layer));
+    }
+
+    [Fact]
+    public void AddArc_EngraveLayer_TagsScribe()
+    {
+        var arc = new Arc(new Vector(0, 0), 1, 0, System.Math.PI / 2) { Layer = new Layer("ENGRAVE") };
+
+        var pgm = ProgramFor(arc);
+
+        var arcs = pgm.Codes.OfType<ArcMove>().ToList();
+        Assert.NotEmpty(arcs);
+        Assert.All(arcs, m => Assert.Equal(LayerType.Scribe, m.Layer));
+    }
+
+    [Fact]
+    public void AddCircle_EngraveLayer_TagsScribe()
+    {
+        var circle = new Circle(0, 0, 1) { Layer = new Layer("ENGRAVE") };
+
+        var pgm = ProgramFor(circle);
+
+        var arcs = pgm.Codes.OfType<ArcMove>().ToList();
+        Assert.NotEmpty(arcs);
+        Assert.All(arcs, m => Assert.Equal(LayerType.Scribe, m.Layer));
+    }
+
+    [Fact]
+    public void AddLine_DefaultLayer_StaysCut()
+    {
+        var line = new Line(0, 0, 1, 0) { Layer = new Layer("0") };
+
+        var pgm = ProgramFor(line);
+
+        Assert.All(pgm.Codes.OfType<LinearMove>(), m => Assert.Equal(LayerType.Cut, m.Layer));
+    }
+}