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/RapidMove.cs

@@ -1,4 +1,5 @@
-using OpenNest.Geometry;
+using System.Collections.Generic;
+using OpenNest.Geometry;
 
 namespace OpenNest.CNC
 {
@@ -28,7 +29,8 @@ namespace OpenNest.CNC
         {
             return new RapidMove(EndPoint)
             {
-                Suppressed = Suppressed
+                Suppressed = Suppressed,
+                VariableRefs = VariableRefs != null ? new Dictionary<string, string>(VariableRefs) : null
             };
         }