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>

OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj

@@ -1,13 +1,3 @@
 <Project Sdk="Microsoft.NET.Sdk">
-  <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <RootNamespace>OpenNest.Engine.Qwen</RootNamespace>
-    <AssemblyName>OpenNest.Engine.Qwen</AssemblyName>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
-  </PropertyGroup>
-  <ItemGroup>
-    <Compile Remove="tests/**/*.cs" />
-    <ProjectReference Include="../OpenNest.Engine/OpenNest.Engine.csproj" />
-  </ItemGroup>
+  <!-- Shared settings and the OpenNest.Engine reference come from Engines/Directory.Build.props. -->
 </Project>

OpenNest.Engine.Qwen/README.md

@@ -31,7 +31,7 @@ engines to start; it must not be the same algorithm re-derived through indirecti
 ## Build
 
 ```bash
-dotnet build OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj
+dotnet build Engines/OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj
 ```
 
 This project is intentionally **outside** `OpenNest.sln` (same pattern as the
@@ -44,14 +44,16 @@ as part of the main solution.
 own build output:
 
 ```bash
-dotnet build OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj -c Release
+dotnet build Engines/OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj -c Release
 dotnet build OpenNest.Benchmark/OpenNest.Benchmark.csproj -c Release
 
 mkdir -p OpenNest.Benchmark/bin/Release/net8.0/Engines
-cp OpenNest.Engine.Qwen/bin/Release/net8.0/OpenNest.Engine.Qwen.dll    OpenNest.Benchmark/bin/Release/net8.0/Engines/
+cp Engines/OpenNest.Engine.Qwen/bin/Release/net8.0/OpenNest.Engine.Qwen.dll    OpenNest.Benchmark/bin/Release/net8.0/Engines/
 
 dotnet OpenNest.Benchmark/bin/Release/net8.0/OpenNest.Benchmark.dll <path-to-.nest-or-folder>
 ```
 
+Or build and deploy in one step with `./Engines/Build-Engines.ps1 -Engines Qwen`.
+
 Your engine will show up in the report under its CLR type name (`QwenNestingEngine`),
 competing on equal footing against the built-in engines.

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

@@ -1,8 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
   </PropertyGroup>
@@ -14,6 +11,5 @@
   <ItemGroup>
     <Using Include="Xunit" />
     <ProjectReference Include="../OpenNest.Engine.Qwen.csproj" />
-    <ProjectReference Include="../../OpenNest.Engine/OpenNest.Engine.csproj" />
   </ItemGroup>
 </Project>