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/Trigonometry.cs

@@ -0,0 +1,40 @@
+using System;
+
+namespace OpenNest
+{
+    public static class Trigonometry
+    {
+        /// <summary>
+        ///
+        /// </summary>
+        /// <param name="height">Height</param>
+        /// <param name="hypotenuse">Hypotenuse</param>
+        /// <returns></returns>
+        public static double Base(double height, double hypotenuse)
+        {
+            return Math.Sqrt(hypotenuse * hypotenuse - height * height);
+        }
+
+        /// <summary>
+        ///
+        /// </summary>
+        /// <param name="bottom">Base</param>
+        /// <param name="hypotenuse">Hypotenuse</param>
+        /// <returns></returns>
+        public static double Height(double bottom, double hypotenuse)
+        {
+            return Math.Sqrt(hypotenuse * hypotenuse - bottom * bottom);
+        }
+
+        /// <summary>
+        ///
+        /// </summary>
+        /// <param name="height">Height</param>
+        /// <param name="bottom">Base</param>
+        /// <returns></returns>
+        public static double Hypotenuse(double height, double bottom)
+        {
+            return Math.Sqrt(height * height + bottom * bottom);
+        }
+    }
+}