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/PackFirstFitDecreasing.cs

@@ -0,0 +1,121 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace OpenNest.RectanglePacking
+{
+    internal class FirstFitDecreasing : PackEngine
+    {
+        private readonly List<Level> levels;
+
+        public FirstFitDecreasing(Bin bin)
+            : base(bin)
+        {
+            levels = new List<Level>();
+        }
+
+        public override void Pack(List<Item> items)
+        {
+            items = items.OrderBy(i => -i.Height).ToList();
+
+            foreach (var item in items)
+            {
+                if (item.Height > Bin.Height)
+                    continue;
+
+                var level = FindLevel(item);
+
+                if (level == null)
+                    continue;
+
+                level.AddItem(item);
+            }
+        }
+
+        private Level FindLevel(Item item)
+        {
+            foreach (var level in levels)
+            {
+                if (level.Height < item.Height)
+                    continue;
+
+                if (level.RemainingWidth < item.Width)
+                    continue;
+
+                return level;
+            }
+
+            return CreateNewLevel(item);
+        }
+
+        private Level CreateNewLevel(Item item)
+        {
+            var y = Bin.Y;
+            var lastLevel = levels.LastOrDefault();
+
+            if (lastLevel != null)
+                y = lastLevel.Y + lastLevel.Height;
+
+            var remaining = Bin.Top - y;
+
+            if (remaining < item.Height)
+                return null;
+
+            var level = new Level(Bin);
+            level.Y = y;
+            level.Height = item.Height;
+
+            levels.Add(level);
+
+            return level;
+        }
+
+        private class Level
+        {
+            public Level(Bin parent)
+            {
+                Parent = parent;
+                NextItemLocation = parent.Location;
+            }
+
+            public Bin Parent { get; set; }
+
+            private Vector NextItemLocation;
+
+            public double X
+            {
+                get { return Parent.X; }
+            }
+
+            public double Y
+            {
+                get { return NextItemLocation.Y; }
+                set { NextItemLocation.Y = value; }
+            }
+
+            public double Width
+            {
+                get { return Parent.Width; }
+            }
+
+            public double Height { get; set; }
+
+            public double Top
+            {
+                get { return Y + Height; }
+            }
+
+            public double RemainingWidth
+            {
+                get { return X + Width - NextItemLocation.X; }
+            }
+
+            public void AddItem(Item item)
+            {
+                item.Location = NextItemLocation;
+                Parent.Items.Add(item);
+
+                NextItemLocation = new Vector(NextItemLocation.X + item.Width, NextItemLocation.Y);
+            }
+        }
+    }
+}