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.Data/EngineSelectionSettings.cs

@@ -46,14 +46,20 @@ public sealed class EngineSelectionSettings
     /// <summary>
     /// Resolves against the host's selectable engines AFTER plug-in loading. A missing engine
     /// returns Default plus a status-bar warning, without replacing the saved preference.
-    /// Names use registry casing so desktop combo-box selection remains exact.
+    /// Names use registry casing so desktop combo-box selection remains exact. When the saved
+    /// name is not selectable, <paramref name="renamed"/> (the registry's legacy-name lookup)
+    /// may map it to the engine that replaced it; the result must itself be selectable.
     /// </summary>
-    public string Resolve(IEnumerable<string> availableEngineNames, out string? statusMessage)
+    public string Resolve(
+        IEnumerable<string> availableEngineNames,
+        out string? statusMessage,
+        Func<string, string?>? renamed = null)
     {
         ArgumentNullException.ThrowIfNull(availableEngineNames);
+        var available = availableEngineNames.ToList();
         var requestedName = NormalizeName(EngineName);
-        var registeredName = availableEngineNames.FirstOrDefault(
-            name => string.Equals(name, requestedName, StringComparison.OrdinalIgnoreCase));
+        var registeredName = Find(available, requestedName)
+            ?? (renamed?.Invoke(requestedName) is { } replacement ? Find(available, replacement) : null);
         if (registeredName is not null)
         {
             statusMessage = null;
@@ -64,6 +70,9 @@ public sealed class EngineSelectionSettings
         return DefaultEngineName;
     }
 
+    private static string? Find(IEnumerable<string> names, string name) =>
+        names.FirstOrDefault(n => string.Equals(n, name, StringComparison.OrdinalIgnoreCase));
+
     /// <summary>
     /// Writes camelCase JSON, creating the parent directory and retrying IO collisions as
     /// LocalJsonProvider does. The host handles a persistent write failure.