feat: add Variables dictionary to Program with deep-copy in Clone

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

OpenNest.Core/CNC/Program.cs

@@ -1,6 +1,7 @@
 using OpenNest.Converters;
 using OpenNest.Geometry;
 using OpenNest.Math;
+using System;
 using System.Collections.Generic;
 
 namespace OpenNest.CNC
@@ -9,6 +10,8 @@ namespace OpenNest.CNC
     {
         public List<ICode> Codes;
 
+        public Dictionary<string, VariableDefinition> Variables { get; } = new(StringComparer.OrdinalIgnoreCase);
+
         private Mode mode;
 
         public Program(Mode mode = Mode.Absolute)
@@ -454,6 +457,9 @@ namespace OpenNest.CNC
 
             pgm.Codes.AddRange(codes);
 
+            foreach (var kvp in Variables)
+                pgm.Variables[kvp.Key] = kvp.Value;
+
             return pgm;
         }