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>

OpenNest.Engine.Astra/OpenNest.Engine.Astra.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <ItemGroup>
     <Compile Remove="tests/**/*.cs;benchmarks/**/*.cs" />
-    <ProjectReference Include="../../OpenNest.Core/OpenNest.Core.csproj" />
+    <ProjectReference Include="$(OpenNestRoot)OpenNest.Core/OpenNest.Core.csproj" />
   </ItemGroup>
 </Project>