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.Tests/Data/EngineSelectionSettingsTests.cs

@@ -167,6 +167,31 @@ public class EngineSelectionSettingsTests : IDisposable
         Assert.Null(message);
     }
 
+    [Fact]
+    public void Resolve_RenamedEngineMapsToItsReplacementWithoutRewritingSavedChoice()
+    {
+        new EngineSelectionSettings { EngineName = "Opus55NestingEngine" }.Save(_path);
+        var settings = EngineSelectionSettings.Load(_path);
+
+        var resolved = settings.Resolve(new[] { "Default", "Irregular" }, out var message,
+            name => name == "Opus55NestingEngine" ? "irregular" : null);
+
+        Assert.Equal("Irregular", resolved);
+        Assert.Null(message);
+        Assert.Equal("Opus55NestingEngine", EngineSelectionSettings.Load(_path).EngineName);
+    }
+
+    [Fact]
+    public void Resolve_RenameToAnUnselectableEngineStillFallsBackWithMessage()
+    {
+        var settings = new EngineSelectionSettings { EngineName = "Opus55NestingEngine" };
+
+        var resolved = settings.Resolve(new[] { "Default", "Strip" }, out var message, _ => "Irregular");
+
+        Assert.Equal("Default", resolved);
+        Assert.Equal("Saved Auto Nest engine 'Opus55NestingEngine' is unavailable. Using Default.", message);
+    }
+
     [Fact]
     public void Save_UnwritablePathReportsFailureToCaller()
     {