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/GravographISPostProcessorTests.cs

@@ -0,0 +1,70 @@
+using System.Collections.Generic;
+using OpenNest.CNC;
+using OpenNest.Geometry;
+using OpenNest.Posts.GravographIS;
+
+namespace OpenNest.Tests.GravographIS;
+
+public class GravographISPostProcessorTests
+{
+    private static LayeredPolyline Poly(LayerType layer, params Vector[] pts)
+        => new LayeredPolyline(new List<Vector>(pts), layer);
+
+    [Fact]
+    public void BuildPasses_EngraveAndCut_OrdersEngraveFirstThenCutWithPause()
+    {
+        var post = new GravographISPostProcessor();
+
+        var passes = post.BuildPasses(new[]
+        {
+            Poly(LayerType.Cut, new Vector(0, 0), new Vector(1, 0)),
+            Poly(LayerType.Scribe, new Vector(0, 0), new Vector(0, 1)),
+        });
+
+        Assert.Equal(2, passes.Count);
+
+        Assert.Equal(post.Config.Engrave.FeedMmPerSec, passes[0].FeedMmPerSec);
+        Assert.False(passes[0].PauseBefore);
+
+        Assert.Equal(post.Config.Cut.FeedMmPerSec, passes[1].FeedMmPerSec);
+        Assert.True(passes[1].PauseBefore);
+        Assert.Equal("Change tool", passes[1].PauseMessage);
+    }
+
+    [Fact]
+    public void BuildPasses_CutOnly_IsSinglePass()
+    {
+        var post = new GravographISPostProcessor();
+
+        var passes = post.BuildPasses(new[]
+        {
+            Poly(LayerType.Cut, new Vector(0, 0), new Vector(1, 0)),
+        });
+
+        Assert.Single(passes);
+        Assert.Equal(post.Config.Cut.FeedMmPerSec, passes[0].FeedMmPerSec);
+    }
+
+    [Fact]
+    public void BuildPasses_SkipsDisplayGeometry()
+    {
+        var post = new GravographISPostProcessor();
+
+        var passes = post.BuildPasses(new[]
+        {
+            Poly(LayerType.Display, new Vector(0, 0), new Vector(1, 0)),
+            Poly(LayerType.Cut, new Vector(0, 0), new Vector(0, 1)),
+        });
+
+        Assert.Single(passes);
+        Assert.Equal(post.Config.Cut.FeedMmPerSec, passes[0].FeedMmPerSec);
+    }
+
+    [Fact]
+    public void Config_IsExposedThroughConfigurableInterface()
+    {
+        var post = new GravographISPostProcessor(new GravographISPostConfig());
+        OpenNest.IConfigurablePostProcessor configurable = post;
+        Assert.Same(post.Config, configurable.Config);
+    }
+}