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.Core/BoxSplitter.cs

@@ -0,0 +1,57 @@
+namespace OpenNest
+{
+    public static class BoxSplitter
+    {
+        public static Box SplitTop(Box large, Box small)
+        {
+            if (!large.Intersects(small))
+                return Box.Empty;
+
+            var x = large.Left;
+            var y = small.Top;
+            var w = large.Width;
+            var h = large.Top - y;
+
+            return new Box(x, y, w, h);
+        }
+
+        public static Box SplitLeft(Box large, Box small)
+        {
+            if (!large.Intersects(small))
+                return Box.Empty;
+
+            var x = large.Left;
+            var y = large.Bottom;
+            var w = small.Left - x;
+            var h = large.Height;
+
+            return new Box(x, y, w, h);
+        }
+
+        public static Box SplitBottom(Box large, Box small)
+        {
+            if (!large.Intersects(small))
+                return Box.Empty;
+
+            var x = large.Left;
+            var y = large.Bottom;
+            var w = large.Width;
+            var h = small.Top - y;
+
+            return new Box(x, y, w, h);
+        }
+
+        public static Box SplitRight(Box large, Box small)
+        {
+            if (!large.Intersects(small))
+                return Box.Empty;
+
+            var x = small.Right;
+            var y = large.Bottom;
+            var w = large.Right - x;
+            var h = large.Height;
+
+            return new Box(x, y, w, h);
+        }
+    }
+}