feat: add VariableDefinition type for G-code user variables

Adds immutable VariableDefinition record to OpenNest.CNC with name,
expression, resolved value, inline, and global flags. Fixes namespace
collision in PatternTilerTests and PolygonHelperTests caused by the new
OpenNest.Tests.CNC namespace.

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

OpenNest.Core/CNC/VariableDefinition.cs

@@ -0,0 +1,21 @@
+namespace OpenNest.CNC
+{
+    public sealed class VariableDefinition
+    {
+        public string Name { get; }
+        public string Expression { get; }
+        public double Value { get; }
+        public bool Inline { get; }
+        public bool Global { get; }
+
+        public VariableDefinition(string name, string expression, double value,
+            bool inline = false, bool global = false)
+        {
+            Name = name;
+            Expression = expression;
+            Value = value;
+            Inline = inline;
+            Global = global;
+        }
+    }
+}

OpenNest.Tests/CNC/VariableDefinitionTests.cs

@@ -0,0 +1,33 @@
+using OpenNest.CNC;
+
+namespace OpenNest.Tests.CNC;
+
+public class VariableDefinitionTests
+{
+    [Fact]
+    public void Constructor_SetsAllProperties()
+    {
+        var v = new VariableDefinition("diameter", "0.3", 0.3);
+        Assert.Equal("diameter", v.Name);
+        Assert.Equal("0.3", v.Expression);
+        Assert.Equal(0.3, v.Value);
+        Assert.False(v.Inline);
+        Assert.False(v.Global);
+    }
+
+    [Fact]
+    public void Constructor_WithFlags_SetsFlags()
+    {
+        var v = new VariableDefinition("width", "48.0", 48.0, inline: true, global: true);
+        Assert.True(v.Inline);
+        Assert.True(v.Global);
+    }
+
+    [Fact]
+    public void DefaultFlags_AreFalse()
+    {
+        var v = new VariableDefinition("x", "1", 1.0);
+        Assert.False(v.Inline);
+        Assert.False(v.Global);
+    }
+}

OpenNest.Tests/Fill/PatternTilerTests.cs

@@ -1,3 +1,4 @@
+using OpenNest.CNC;
 using OpenNest.Engine.Fill;
 using OpenNest.Geometry;
 
@@ -7,11 +8,11 @@ public class PatternTilerTests
 {
     private static Drawing MakeSquareDrawing(double size)
     {
-        var pgm = new CNC.Program();
-        pgm.Codes.Add(new CNC.LinearMove(size, 0));
-        pgm.Codes.Add(new CNC.LinearMove(size, size));
-        pgm.Codes.Add(new CNC.LinearMove(0, size));
-        pgm.Codes.Add(new CNC.LinearMove(0, 0));
+        var pgm = new Program();
+        pgm.Codes.Add(new LinearMove(size, 0));
+        pgm.Codes.Add(new LinearMove(size, size));
+        pgm.Codes.Add(new LinearMove(0, size));
+        pgm.Codes.Add(new LinearMove(0, 0));
         return new Drawing("square", pgm);
     }
 

OpenNest.Tests/Geometry/PolygonHelperTests.cs

@@ -57,12 +57,12 @@ public class PolygonHelperTests
     public void ExtractPerimeterPolygon_InflatedPolygonIsLarger_ForCCWWinding()
     {
         // CCW winding: (0,0)→(10,0)→(10,10)→(0,10)→(0,0)
-        var pgm = new CNC.Program();
-        pgm.Codes.Add(new CNC.RapidMove(new Vector(0, 0)));
-        pgm.Codes.Add(new CNC.LinearMove(new Vector(10, 0)));
-        pgm.Codes.Add(new CNC.LinearMove(new Vector(10, 10)));
-        pgm.Codes.Add(new CNC.LinearMove(new Vector(0, 10)));
-        pgm.Codes.Add(new CNC.LinearMove(new Vector(0, 0)));
+        var pgm = new Program();
+        pgm.Codes.Add(new RapidMove(new Vector(0, 0)));
+        pgm.Codes.Add(new LinearMove(new Vector(10, 0)));
+        pgm.Codes.Add(new LinearMove(new Vector(10, 10)));
+        pgm.Codes.Add(new LinearMove(new Vector(0, 10)));
+        pgm.Codes.Add(new LinearMove(new Vector(0, 0)));
         var drawing = new Drawing("ccw-square", pgm);
 
         var noSpacing = PolygonHelper.ExtractPerimeterPolygon(drawing, 0);