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.targets

@@ -0,0 +1,10 @@
+<Project>
+  <!--
+    Each engine's tests/ subproject sits inside the engine folder. This must live in a
+    .targets file: removals in Directory.Build.props run before the SDK adds its default
+    Compile glob, so they would have no effect.
+  -->
+  <ItemGroup>
+    <Compile Remove="tests/**/*.cs" />
+  </ItemGroup>
+</Project>