build: stand alone from the OpenNest repo

Engines are moving out of OpenNest so they can be published on their own
and so OpenNest checkouts handed to a model for an engine-building run no
longer contain the competing engines. History for Engines/ (and the
pre-move root-level engine folders) was carried over with git filter-repo.

Engines now build against a sibling OpenNest clone via $(OpenNestRoot),
overridable with /p:OpenNestRoot, with a clear error when it is missing.

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

Directory.Build.targets

@@ -7,4 +7,9 @@
   <ItemGroup>
     <Compile Remove="tests/**/*.cs" />
   </ItemGroup>
+
+  <Target Name="CheckOpenNestRoot" BeforeTargets="ResolveProjectReferences;_CheckForInvalidConfigurationAndPlatform">
+    <Error Condition="!Exists('$(OpenNestRoot)OpenNest.Engine/OpenNest.Engine.csproj')"
+           Text="OpenNest checkout not found at '$(OpenNestRoot)'. Clone https://git.thecozycat.net/aj/OpenNest.git next to this repo, or pass /p:OpenNestRoot=&lt;path&gt;." />
+  </Target>
 </Project>