feat: add _Template and New-Engine.ps1 for starting new engines

Each engine-building run started from a hand-copied scaffold (Qwen's),
which carried that engine's name and stale notes. _Template holds the
generic scaffold with a __NAME__ placeholder, and New-Engine.ps1 stamps
out a named copy. The template's starter tests go through the
benchmark's NestValidator, so a model gets a real pass/fail target
instead of a plumbing-only check; they were verified to pass against
Opus55.

Directory.Build.props now also detects when it sits in an Engines/
folder inside an OpenNest checkout, so an engine stamped there with
-IncludeBuildFiles builds without passing OpenNestRoot.

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

Directory.Build.props

@@ -4,11 +4,13 @@
     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.
+    Engines build against an OpenNest checkout, found in this order: /p:OpenNestRoot=<path>
+    or an OpenNestRoot environment variable; the parent folder, when these build files sit
+    in an Engines/ folder inside an OpenNest checkout; otherwise a sibling clone
+    (../OpenNest next to this repo).
   -->
   <PropertyGroup>
+    <OpenNestRoot Condition="'$(OpenNestRoot)' == '' and Exists('$(MSBuildThisFileDirectory)../OpenNest.Engine/OpenNest.Engine.csproj')">$(MSBuildThisFileDirectory)../</OpenNestRoot>
     <OpenNestRoot Condition="'$(OpenNestRoot)' == ''">$(MSBuildThisFileDirectory)../OpenNest/</OpenNestRoot>
     <OpenNestRoot>$([MSBuild]::EnsureTrailingSlash('$(OpenNestRoot)'))</OpenNestRoot>
     <TargetFramework>net8.0</TargetFramework>