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/Tolerance.cs

@@ -0,0 +1,14 @@
+using System;
+
+namespace OpenNest
+{
+    public static class Tolerance
+    {
+        public const double Epsilon = 0.00001;
+
+        public static bool IsEqualTo(this double a, double b, double tolerance = Epsilon)
+        {
+            return Math.Abs(b - a) <= tolerance;
+        }
+    }
+}