feat: serialize variable definitions and \$references in NestWriter

Emit variable definitions before G-code in program text entries and use
\$varName syntax for coordinate fields that have VariableRefs, so programs
round-trip through NestWriter → NestReader without losing variable information.

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

OpenNest.Tests/IO/NestWriterVariableTests.cs

@@ -0,0 +1,91 @@
+using System.IO;
+using System.Linq;
+using System.Text;
+using OpenNest.CNC;
+using OpenNest.Geometry;
+using OpenNest.IO;
+
+namespace OpenNest.Tests.IO;
+
+public class NestWriterVariableTests
+{
+    [Fact]
+    public void RoundTrip_VariableDefinitions_Preserved()
+    {
+        var nest = CreateNestWithVariableProgram(
+            "width = 48.0 global\ndiameter = 0.3\nG90\nG01X$widthY$diameter");
+
+        var loaded = RoundTrip(nest);
+        var pgm = loaded.Drawings.First().Program;
+
+        Assert.Equal(2, pgm.Variables.Count);
+        Assert.Equal(48.0, pgm.Variables["width"].Value);
+        Assert.True(pgm.Variables["width"].Global);
+        Assert.Equal(0.3, pgm.Variables["diameter"].Value);
+        Assert.False(pgm.Variables["diameter"].Global);
+    }
+
+    [Fact]
+    public void RoundTrip_VariableRefs_Preserved()
+    {
+        var nest = CreateNestWithVariableProgram(
+            "width = 48.0\nG90\nG01X$widthY0");
+
+        var loaded = RoundTrip(nest);
+        var pgm = loaded.Drawings.First().Program;
+
+        var linear = (LinearMove)pgm.Codes[0];
+        Assert.Equal(48.0, linear.EndPoint.X);
+        Assert.NotNull(linear.VariableRefs);
+        Assert.Equal("width", linear.VariableRefs["X"]);
+    }
+
+    [Fact]
+    public void RoundTrip_InlineFlag_Preserved()
+    {
+        var nest = CreateNestWithVariableProgram(
+            "kerf = 0.06 inline\nG90\nG01X1Y0");
+
+        var loaded = RoundTrip(nest);
+        var pgm = loaded.Drawings.First().Program;
+
+        Assert.True(pgm.Variables["kerf"].Inline);
+    }
+
+    [Fact]
+    public void RoundTrip_NoVariables_WorksAsNormal()
+    {
+        var nest = CreateNestWithVariableProgram("G90\nG01X1Y2");
+
+        var loaded = RoundTrip(nest);
+        var pgm = loaded.Drawings.First().Program;
+
+        Assert.Empty(pgm.Variables);
+        var linear = (LinearMove)pgm.Codes[0];
+        Assert.Equal(1.0, linear.EndPoint.X);
+    }
+
+    private static Nest CreateNestWithVariableProgram(string gcode)
+    {
+        var stream = new MemoryStream(Encoding.UTF8.GetBytes(gcode));
+        var reader = new ProgramReader(stream);
+        var program = reader.Read();
+        reader.Close();
+
+        var drawing = new Drawing("TestPart", program);
+        var nest = new Nest { Name = "Test" };
+        nest.Drawings.Add(drawing);
+        var plate = new Plate(new Size(100, 100));
+        plate.Parts.Add(new Part(drawing, new Vector(0, 0)));
+        nest.Plates.Add(plate);
+        return nest;
+    }
+
+    private static Nest RoundTrip(Nest nest)
+    {
+        var ms = new MemoryStream();
+        new NestWriter(nest).Write(ms);
+        ms.Position = 0;
+        return new NestReader(ms).Read();
+    }
+}