Restructure project layout to flatten directory structure

Move all projects from Source/ to repository root for simpler navigation.
- Remove External/ dependency DLLs (will use NuGet packages)
- Remove Installer/ NSIS script
- Replace PartCollection/PlateCollection with ObservableList
- Add packages.config for NuGet dependencies

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

OpenNest.Engine/RectanglePacking/FillEngine.cs

@@ -0,0 +1,82 @@
+using System.Collections.Generic;
+
+namespace OpenNest.RectanglePacking
+{
+    internal abstract class FillEngine
+    {
+        public FillEngine(Bin bin)
+        {
+            Bin = bin;
+        }
+
+        public Bin Bin { get; set; }
+
+        public abstract void Fill(Item item);
+
+        public abstract void Fill(Item item, int maxCount);
+
+        /// <summary>
+        /// Vertical pattern.
+        /// </summary>
+        /// <param name="item"></param>
+        /// <param name="rows"></param>
+        /// <param name="columns"></param>
+        /// <param name="maxCount"></param>
+        protected List<Item> VPattern(Item item, int rows, int columns, int maxCount)
+        {
+            var items = new List<Item>();
+
+            for (int i = 0; i < columns; i++)
+            {
+                var x = item.Width * i + item.X;
+
+                for (int j = 0; j < rows; j++)
+                {
+                    var y = item.Height * j + item.Y;
+
+                    var addedItem = item.Clone() as Item;
+                    addedItem.Location = new Vector(x, y);
+
+                    items.Add(addedItem);
+
+                    if (items.Count == maxCount)
+                        return items;
+                }
+            }
+
+            return items;
+        }
+
+        /// <summary>
+        /// Horizontal pattern.
+        /// </summary>
+        /// <param name="item"></param>
+        /// <param name="rows"></param>
+        /// <param name="columns"></param>
+        /// <param name="maxCount"></param>
+        protected List<Item> HPattern(Item item, int rows, int columns, int maxCount)
+        {
+            var items = new List<Item>();
+
+            for (int i = 0; i < rows; i++)
+            {
+                var y = item.Height * i + item.Y;
+
+                for (int j = 0; j < rows; j++)
+                {
+                    var x = item.Width * j + item.X;
+
+                    var addedItem = item.Clone() as Item;
+                    addedItem.Location = new Vector(x, y);
+
+                    items.Add(addedItem);
+
+                    if (items.Count == maxCount)
+                        return items;
+                }
+            }
+
+            return items;
+        }
+    }
+}