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/ArcMove.cs

@@ -1,4 +1,5 @@
-using OpenNest.Geometry;
+using System.Collections.Generic;
+using OpenNest.Geometry;
 
 namespace OpenNest.CNC
 {
@@ -66,7 +67,8 @@ namespace OpenNest.CNC
             return new ArcMove(EndPoint, CenterPoint, Rotation)
             {
                 Layer = Layer,
-                Suppressed = Suppressed
+                Suppressed = Suppressed,
+                VariableRefs = VariableRefs != null ? new Dictionary<string, string>(VariableRefs) : null
             };
         }