refactor(engines): move plugin engines into an Engines/ subfolder

Plugin engines (Opus55, Qwen, Terra) each add two projects at the repo
root, and more are coming; at a dozen they would outnumber the core
projects. They are also a different kind of thing: out-of-solution,
runtime-loaded plugins. Grouping them under Engines/ keeps the root
readable.

Engines/Directory.Build.props now holds the shared TFM, nullable and
implicit-usings settings and the OpenNest.Engine reference, so a new
engine's csproj is nearly empty. The tests/ compile exclusion lives in
Directory.Build.targets because a removal in .props runs before the SDK
adds its default Compile glob and has no effect.

Build-Engines.ps1 replaces the per-README manual build-and-copy steps
for deploying engines into the benchmark's runtime Engines/ folder.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Engines/OpenNest.Engine.Qwen/tests/OpenNest.Engine.Qwen.Tests.csproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
+    <PackageReference Include="xunit" Version="2.5.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.3" />
+  </ItemGroup>
+  <ItemGroup>
+    <Using Include="Xunit" />
+    <ProjectReference Include="../OpenNest.Engine.Qwen.csproj" />
+  </ItemGroup>
+</Project>

Engines/OpenNest.Engine.Qwen/tests/QwenNestingEngineTests.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using OpenNest.Engine.Jobs;
+using OpenNest.Geometry;
+
+namespace OpenNest.Engine.Qwen.Tests;
+
+public class QwenNestingEngineTests
+{
+    [Fact]
+    public void SolveReturnsAResultForASingleSimplePart()
+    {
+        // TODO: replace with a real fixture once Solve() is implemented — this only
+        // proves the plumbing (project reference, constructor, interface) is wired up.
+        var engine = new QwenNestingEngine();
+
+        Assert.NotNull(engine);
+        Assert.IsAssignableFrom<INestingEngine>(engine);
+    }
+}