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/FillSameRotation.cs

@@ -0,0 +1,65 @@
+
+namespace OpenNest.RectanglePacking
+{
+    internal class FillSameRotation : FillEngine
+    {
+        public FillSameRotation(Bin bin)
+            : base(bin)
+        {
+        }
+
+        public override void Fill(Item item)
+        {
+            var bin1 = Bin.Clone() as Bin;
+            var bin2 = Bin.Clone() as Bin;
+
+            var engine = new FillNoRotation(bin1);
+            engine.Fill(item);
+
+            item.Rotate();
+
+            engine.Bin = bin2;
+            engine.Fill(item);
+
+            var density1 = bin1.Density();
+            var density2 = bin2.Density();
+
+            if (density1.IsEqualTo(density2))
+            {
+                var bounds1 = bin1.Items.GetBoundingBox();
+                var bounds2 = bin2.Items.GetBoundingBox();
+
+                if (bounds2.Right < bounds1.Right)
+                    Bin.Items.AddRange(bin2.Items);
+                else
+                    Bin.Items.AddRange(bin1.Items);
+            }
+            else if (density1 > density2)
+                Bin.Items.AddRange(bin1.Items);
+            else
+                Bin.Items.AddRange(bin2.Items);
+        }
+
+        public override void Fill(Item item, int maxCount)
+        {
+            var bin1 = Bin.Clone() as Bin;
+            var bin2 = Bin.Clone() as Bin;
+
+            var engine = new FillNoRotation(bin1);
+            engine.Fill(item, maxCount);
+
+            item.Rotate();
+
+            engine.Bin = bin2;
+            engine.Fill(item, maxCount);
+
+            var bounds1 = bin1.Items.GetBoundingBox();
+            var bounds2 = bin2.Items.GetBoundingBox();
+
+            if (bounds2.Right < bounds1.Right)
+                Bin.Items.AddRange(bin2.Items);
+            else
+                Bin.Items.AddRange(bin1.Items);
+        }
+    }
+}