refactor: switch auto-nest from Pack to per-item Fill with explicit quantity tracking

Replace engine.Pack(items) with individual engine.Fill(item) calls so each
drawing is filled independently. Quantity decrements now count parts actually
placed per plate (grouped by drawing) instead of relying on the drawing's
internal remaining counter.

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

OpenNest/Forms/MainForm.cs

@@ -680,26 +680,42 @@ namespace OpenNest.Forms
                 return;
 
             var items = form.GetNestItems();
-            var qty = new int[items.Count];
 
-            while (true)
+            while (items.Any(it => it.Quantity > 0))
             {
-                for (int i = 0; i < items.Count; i++)
-                    qty[i] = items[i].Drawing.Quantity.Remaining;
-
                 var plate = activeForm.PlateView.Plate.Parts.Count > 0
                     ? activeForm.Nest.CreatePlate()
                     : activeForm.PlateView.Plate;
 
                 var engine = new NestEngine(plate);
+                var filled = false;
 
-                if (!engine.Pack(items))
+                foreach (var item in items)
+                {
+                    if (item.Quantity <= 0)
+                        continue;
+
+                    if (engine.Fill(item))
+                        filled = true;
+                }
+
+                if (!filled)
                     break;
 
-                activeForm.Nest.UpdateDrawingQuantities();
+                // Decrement requested quantities by counting parts actually
+                // placed on this plate, grouped by drawing.
+                foreach (var group in plate.Parts.GroupBy(p => p.BaseDrawing))
+                {
+                    var placed = group.Count();
 
-                for (int i = 0; i < items.Count; i++)
-                    items[i].Quantity -= qty[i] - items[i].Drawing.Quantity.Remaining;
+                    foreach (var item in items)
+                    {
+                        if (item.Drawing == group.Key)
+                            item.Quantity -= placed;
+                    }
+                }
+
+                activeForm.Nest.UpdateDrawingQuantities();
             }
         }