feat: unify ActionAddPart into ActionClone and add group fill support

Merge ActionAddPart into ActionClone by adding a Drawing constructor,
eliminating the redundant class. ActionClone now handles both adding
new parts from a drawing and cloning selected part groups. Added
Ctrl+F fill support for groups using FillLinear pattern tiling, and
adopted quadrant-aware push directions from ActionAddPart. Refactored
FillLinear to extract shared helpers and add a Fill(Pattern) overload
for tiling arbitrary part groups across the work area.

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

OpenNest/Forms/EditNestForm.cs

@@ -687,7 +687,7 @@ namespace OpenNest.Forms
             if (drawing == null)
                 return;
 
-            PlateView.SetAction(typeof(ActionAddPart), drawing);
+            PlateView.SetAction(typeof(ActionClone), drawing);
 
             addPart = false;
         }