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/CNC/Feedrate.cs

@@ -0,0 +1,35 @@
+namespace OpenNest.CNC
+{
+    public class Feedrate : ICode
+    {
+        public const int UseDefault = -1;
+
+        public const int UseMax = -2;
+
+        public Feedrate()
+        {
+        }
+
+        public Feedrate(double value)
+        {
+            Value = value;
+        }
+
+        public double Value { get; set; }
+
+        public CodeType Type
+        {
+            get { return CodeType.SetFeedrate; }
+        }
+
+        public ICode Clone()
+        {
+            return new Feedrate(Value);
+        }
+
+        public override string ToString()
+        {
+            return string.Format("F{0}", Value);
+        }
+    }
+}