feat: add VariableRefs tracking on Motion and Feedrate

Adds Dictionary<string,string> VariableRefs to Motion (cleared on Rotate/Offset) and string VariableRef to Feedrate, with deep-copy Clone() support, so post processors can emit variable references instead of literal coordinate values.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Core/CNC/Feedrate.cs

@@ -17,6 +17,8 @@
 
         public double Value { get; set; }
 
+        public string VariableRef { get; set; }
+
         public CodeType Type
         {
             get { return CodeType.SetFeedrate; }
@@ -24,7 +26,7 @@
 
         public ICode Clone()
         {
-            return new Feedrate(Value);
+            return new Feedrate(Value) { VariableRef = VariableRef };
         }
 
         public override string ToString()