feat: add NestEngineInfo and NestEngineRegistry with plugin loading

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/NestEngineInfo.cs

@@ -0,0 +1,18 @@
+using System;
+
+namespace OpenNest
+{
+    public class NestEngineInfo
+    {
+        public NestEngineInfo(string name, string description, Func<Plate, NestEngineBase> factory)
+        {
+            Name = name;
+            Description = description;
+            Factory = factory;
+        }
+
+        public string Name { get; }
+        public string Description { get; }
+        public Func<Plate, NestEngineBase> Factory { get; }
+    }
+}

OpenNest.Engine/NestEngineRegistry.cs

@@ -0,0 +1,96 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Linq;
+using System.Reflection;
+
+namespace OpenNest
+{
+    public static class NestEngineRegistry
+    {
+        private static readonly List<NestEngineInfo> engines = new();
+
+        static NestEngineRegistry()
+        {
+            Register("Default",
+                "Multi-phase nesting (Linear, Pairs, RectBestFit, Remainder)",
+                plate => new DefaultNestEngine(plate));
+        }
+
+        public static IReadOnlyList<NestEngineInfo> AvailableEngines => engines;
+
+        public static string ActiveEngineName { get; set; } = "Default";
+
+        public static NestEngineBase Create(Plate plate)
+        {
+            var info = engines.FirstOrDefault(e =>
+                e.Name.Equals(ActiveEngineName, StringComparison.OrdinalIgnoreCase));
+
+            if (info == null)
+            {
+                Debug.WriteLine($"[NestEngineRegistry] Engine '{ActiveEngineName}' not found, falling back to Default");
+                info = engines[0];
+            }
+
+            return info.Factory(plate);
+        }
+
+        public static void Register(string name, string description, Func<Plate, NestEngineBase> factory)
+        {
+            if (engines.Any(e => e.Name.Equals(name, StringComparison.OrdinalIgnoreCase)))
+            {
+                Debug.WriteLine($"[NestEngineRegistry] Duplicate engine '{name}' skipped");
+                return;
+            }
+
+            engines.Add(new NestEngineInfo(name, description, factory));
+        }
+
+        public static void LoadPlugins(string directory)
+        {
+            if (!Directory.Exists(directory))
+                return;
+
+            foreach (var dll in Directory.GetFiles(directory, "*.dll"))
+            {
+                try
+                {
+                    var assembly = Assembly.LoadFrom(dll);
+
+                    foreach (var type in assembly.GetTypes())
+                    {
+                        if (type.IsAbstract || !typeof(NestEngineBase).IsAssignableFrom(type))
+                            continue;
+
+                        var ctor = type.GetConstructor(new[] { typeof(Plate) });
+
+                        if (ctor == null)
+                        {
+                            Debug.WriteLine($"[NestEngineRegistry] Skipping {type.Name}: no Plate constructor");
+                            continue;
+                        }
+
+                        // Create a temporary instance to read Name and Description.
+                        try
+                        {
+                            var tempPlate = new Plate();
+                            var instance = (NestEngineBase)ctor.Invoke(new object[] { tempPlate });
+                            Register(instance.Name, instance.Description,
+                                plate => (NestEngineBase)ctor.Invoke(new object[] { plate }));
+                            Debug.WriteLine($"[NestEngineRegistry] Loaded plugin engine: {instance.Name}");
+                        }
+                        catch (Exception ex)
+                        {
+                            Debug.WriteLine($"[NestEngineRegistry] Failed to instantiate {type.Name}: {ex.Message}");
+                        }
+                    }
+                }
+                catch (Exception ex)
+                {
+                    Debug.WriteLine($"[NestEngineRegistry] Failed to load assembly {Path.GetFileName(dll)}: {ex.Message}");
+                }
+            }
+        }
+    }
+}