Rename Quantity to DwgQty

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

OpenNest.Core/Drawing.cs

@@ -33,7 +33,7 @@ namespace OpenNest
 
         public int Priority { get; set; }
 
-        public Quantity Quantity;
+        public DwgQty Quantity;
 
         public Material Material { get; set; }
 

OpenNest.Core/DwgQty.cs

@@ -1,6 +1,6 @@
 namespace OpenNest
 {
-    public struct Quantity
+    public struct DwgQty
     {
         public int Nested { get; internal set; }