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/Material.cs

@@ -0,0 +1,33 @@
+namespace OpenNest
+{
+    public class Material
+    {
+        public Material()
+        {
+        }
+
+        public Material(string name)
+        {
+            Name = name;
+        }
+
+        public Material(string name, string grade)
+        {
+            Name = name;
+            Grade = grade;
+        }
+
+        public Material(string name, string grade, double density)
+        {
+            Name = name;
+            Grade = grade;
+            Density = density;
+        }
+
+        public string Name { get; set; }
+
+        public string Grade { get; set; }
+
+        public double Density { get; set; }
+    }
+}