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.props

@@ -1,15 +1,21 @@
 <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.
+    Shared settings for OpenNest nesting engine plugins. Each engine lives in
+    OpenNest.Engine.<Name>/ with an optional tests/ subproject; both import this.
     AssemblyName and RootNamespace default to the project file name.
+
+    Engines build against an OpenNest checkout. By default that is a sibling clone
+    (../OpenNest next to this repo); override with /p:OpenNestRoot=<path> or an
+    OpenNestRoot environment variable.
   -->
   <PropertyGroup>
+    <OpenNestRoot Condition="'$(OpenNestRoot)' == ''">$(MSBuildThisFileDirectory)../OpenNest/</OpenNestRoot>
+    <OpenNestRoot>$([MSBuild]::EnsureTrailingSlash('$(OpenNestRoot)'))</OpenNestRoot>
     <TargetFramework>net8.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
-    <ProjectReference Include="$(MSBuildThisFileDirectory)../OpenNest.Engine/OpenNest.Engine.csproj" />
+    <ProjectReference Include="$(OpenNestRoot)OpenNest.Engine/OpenNest.Engine.csproj" />
   </ItemGroup>
 </Project>