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>

Directory.Build.props

@@ -0,0 +1,15 @@
+<Project>
+  <!--
+    Shared settings for out-of-solution nesting engine plugins. Each engine lives in
+    Engines/OpenNest.Engine.<Name>/ with an optional tests/ subproject; both import this.
+    AssemblyName and RootNamespace default to the project file name.
+  -->
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="$(MSBuildThisFileDirectory)../OpenNest.Engine/OpenNest.Engine.csproj" />
+  </ItemGroup>
+</Project>