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>

New-Engine.ps1

@@ -0,0 +1,49 @@
+<#
+.SYNOPSIS
+    Creates a new engine project from _Template/.
+
+.DESCRIPTION
+    Copies _Template/ to <Destination>/OpenNest.Engine.<Name>/ and replaces the __NAME__
+    placeholder in file names and contents. The new engine builds against OpenNest via
+    Directory.Build.props, which must sit in <Destination> or a parent folder.
+
+    With -IncludeBuildFiles, Directory.Build.props/.targets are copied into <Destination>
+    too, so the engine can live outside this repo, e.g. in an Engines/ folder inside an
+    OpenNest checkout (the props detect that layout on their own).
+
+.EXAMPLE
+    ./New-Engine.ps1 -Name Nova
+    ./New-Engine.ps1 -Name Nova -Destination C:/bench/run1/OpenNest/Engines -IncludeBuildFiles
+#>
+param(
+    [Parameter(Mandatory)]
+    [ValidatePattern('^[A-Z][A-Za-z0-9]*$')]
+    [string]$Name,
+    [string]$Destination = $PSScriptRoot,
+    [switch]$IncludeBuildFiles
+)
+
+$ErrorActionPreference = 'Stop'
+$template = Join-Path $PSScriptRoot '_Template'
+$target = Join-Path $Destination "OpenNest.Engine.$Name"
+if (Test-Path $target) { throw "'$target' already exists." }
+
+New-Item -ItemType Directory -Force $Destination | Out-Null
+Copy-Item $template $target -Recurse
+
+Get-ChildItem $target -Recurse -File | ForEach-Object {
+    $text = [IO.File]::ReadAllText($_.FullName)
+    [IO.File]::WriteAllText($_.FullName, $text.Replace('__NAME__', $Name))
+}
+# Deepest paths first so renaming a folder never invalidates a pending child path.
+Get-ChildItem $target -Recurse | Where-Object Name -like '*__NAME__*' |
+    Sort-Object { $_.FullName.Length } -Descending |
+    ForEach-Object { Rename-Item $_.FullName $_.Name.Replace('__NAME__', $Name) }
+
+if ($IncludeBuildFiles) {
+    foreach ($file in 'Directory.Build.props', 'Directory.Build.targets') {
+        Copy-Item (Join-Path $PSScriptRoot $file) $Destination -Force
+    }
+}
+
+Write-Host "Created $target"