feat(engine): add built-in Rectangles and Irregular nesting engines

Moves the two production plug-in engines into OpenNest.Engine under names
that describe the jobs they suit:

- Rectangles: plain and near-rectangular plates, maximal-rectangles box
  packing (was the RectanglesNestingEngine plug-in)
- Irregular: irregular profiles, no-fit-polygon frontier packing (was the
  Opus55NestingEngine plug-in)

Their tests and the shared engine contract/layout test kit move into
OpenNest.Engine.Tests/NestingEngines.

The registry maps the old plug-in names to the new engines, so saved desktop
selections, scripts and API requests keep working, and a leftover plug-in DLL
under an old name cannot shadow its replacement. Desktop startup passes the
registry's lookup when restoring the saved Auto Nest engine.

OpenNest.Engine.Tests/Jobs/NestingEngineRegistryTests.cs

@@ -1,5 +1,5 @@
-using Xunit;
 using OpenNest.Engine.Jobs;
+using Xunit;
 
 namespace OpenNest.Engine.Tests.Jobs;
 
@@ -10,12 +10,45 @@ public class NestingEngineRegistryTests
     {
         var names = NestingEngineRegistry.AvailableEngines.Select(e => e.Name).ToList();
 
+        Assert.Contains("Rectangles", names);
+        Assert.Contains("Irregular", names);
         Assert.Contains("Default", names);
         Assert.Contains("Strip", names);
         Assert.Contains("Vertical Remnant", names);
         Assert.Contains("Horizontal Remnant", names);
     }
 
+    [Fact]
+    public void RenamedPlugInNamesResolveToTheirBuiltInReplacements()
+    {
+        Assert.Equal("Irregular", NestingEngineRegistry.ResolveName("Opus55NestingEngine"));
+        Assert.Equal("Rectangles", NestingEngineRegistry.ResolveName("rectanglesnestingengine"));
+        Assert.Equal("Rectangles", NestingEngineRegistry.ResolveName(" rectangles "));
+        Assert.IsType<OpenNest.Engine.NestingEngines.Irregular.IrregularNestingEngine>(
+            NestingEngineRegistry.Create("Opus55NestingEngine"));
+    }
+
+    [Fact]
+    public void RetiredEnginesAreNotSilentlyAliased()
+    {
+        Assert.Null(NestingEngineRegistry.ResolveName("Gpt6AstraNestingEngine"));
+        Assert.Null(NestingEngineRegistry.ResolveName("Qwen38FlashNextNestingEngine"));
+        Assert.Null(NestingEngineRegistry.ResolveName("  "));
+        Assert.Throws<NotSupportedException>(() => NestingEngineRegistry.Create("Qwen38FlashNextNestingEngine"));
+    }
+
+    [Fact]
+    public void LeftoverPlugInUnderARenamedNameCannotShadowItsReplacement()
+    {
+        var before = NestingEngineRegistry.AvailableEngines.Count;
+
+        NestingEngineRegistry.Register("Opus55NestingEngine", "stale plug-in",
+            () => new FixedStrategyNestingEngine("Default"));
+
+        Assert.Equal(before, NestingEngineRegistry.AvailableEngines.Count);
+        Assert.Equal("Irregular", NestingEngineRegistry.ResolveName("Opus55NestingEngine"));
+    }
+
     [Fact]
     public void EachBuiltInFactoryProducesAWorkingEngine()
     {